UC-NRLF 


dob 


IA; 

FOR  THK 

5COPICAL  DETERM 

>CK-FORMING  MINERAT  S 
AND  ROCKS 


ALBERT  JOHANNSEN 


-BERKELEY 
LIBRARY 

UNIVERSITY   OF 
CALIFORNIA 


EARTH 

SCIENCE* 

I  I9RARY 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 
OF  ROCK-FORMING  MINERALS  AND  ROCKS 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


THE  BAKER  4  TAYLOR  COMPANY 

MEW  YORK 

THE  CAMBRIDGE  UNIVERSITY  PRESS 

LONDOH 

THE  MARUZEN-KABUSHIKI-KAISHA 

TOKYO,  OSAKA,  KYOTO,  rUKUOKA,  8ENDA1 

THE  MISSION  BOOK  COMPANY 

SHANGHAI 


ESSENTIALS 

FOR  THE 

MICROSCOPICAL  DETERMINATION 

OF 

ROCK-FORMING  MINERALS 
AND  ROCKS 

IN  THIN  SECTIONS 


BY  ALBERT  JOHANNSEN,  PH.D. 

PROFESSOR  OF  PETROLOGY 
THE  UNIVERSITY  OF  CHICAGO 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


'  1  * 


EARTH 


COPYRIGHT  1922  Bv 
THE  UNIVERSITY  OF  CHICAGO 


All  Rights  Reserved 


Published  June  1922 


Composed  and  Printed  By 

The  University  of  Chicago  Press 

Chicago.  Illinois.  U.S.A. 


PREFACE 

This  laboratory  manual  contains  practically  all  of  the  data  originally  published  in  the  writer's 
l>, iirininiition  of  Rock-I-'anninii  Mineral*,  and  in  addition  gives  modes  of  occurrence  and  many  more 
points  of  separation  between  similar  minerals.  Only  a  few  very  rare  species,  such  as  johnstrupite, 
mosaiitlrite,  laavenite.  etc.,  have  been  omitted,  hut  by  uniting  the  tables  which  contained  minerals 
having  birefringences  greater  or  less  than  quartz,  and  refractive  indices  greater  or  less  than  Canada 
balsam,  much  repetition  has  been  avoided,  and  the  number  of  pages  has  Ijcen  materially  reduced. 
t  M-thorhonibic  minerals.  Have  In-en  united  with  the  other  biaxial  minerals,  since  sections  which  cut 
all  of  the  crystallographic  axes  in  this  system  show  inclined  extinction.  The  maximum  extinction 
angles,  of  course,  are  given  in  the  descriptions.  The  index  of  Canada  balsam  is  shown  on  the  dia- 
grams at  l..">:57,  the  mean  of  that  found  in  good  sections. 

The  separation  lines  between  the  various  plagioclasc  feldspars  have  been  changed  from  those 
given  in  the  former  book  to  5,  27$,  50,  72J,  and  95  per  cent  anorthite.  Alhite  and  anorthite  have  been 
limited  to  a  variation  of  only  5  per  cent  since  these  names  are  also  applied  to  the  pure  end  members, 
and  compound  names  such  as  oligoclase-albite,  labradorite-bytownite,  etc.,  have  been  omitted. 

The  section  on  the  determination  of  the  feldspars  has  been  but  little  reduced,  but  that  on  optical 
met  hods  has  been  condensed  as  much  as  possible  since  this  data  is  given  elsewhere.  Manual,  through- 
out the  text,  refers  to  the  author's  Manual  of  Petrographic  Methods,  2d  edition,  New  York,  1918. 
The  alphabetical  list  of  minerals  has  been  much  extended,  and  is  now  placed  at  the  back  of  the  book 
in  such  a  position  that  reference  to  it  may  be  made  without  turning  pages.  Finally,  a  short  section 
on  the  determination  of  rocks  has  been  added,  so  that  when  minerals  and  their  percentages  have  been 
determined,  the  rock  name  also  may  be  known. 

The  writer  has  intentionally  used  words  such  as  "sometimes,"  "often,"  and  "occasionally"  for 
"in  some  specimens,"  "in  many  localities,"  "here  and  there,"  and  so  on,  believing  that  the  use  of 
adverbs  of  time  for  adverbs  of  place,  especially  in  a  book  such  as  this,  which  is  intended  for  laboratory 
use  and  which  therefore  should  Ix:  as  brief  as  possible,  is  justified  by  their  use  in  this  manner  by  many 
of  the  writers  of  the  classics.  Not  only  is  this  usage  customary  in  English,  but  in  foreign  languages 

as  well. 

ALBERT  JOHANNSEN 
UNIVERSITY  or  CHICAGO 
April  18,  1922 


3374 


KEY  TO  THE  TABLES 


Opaque . 


/™*~1  «„!„,, 

Uniaxial  

4 

Odorless  < 

Biaxial  

8 

Anisotropic  

f  Uniaxial  .  .  . 

.     14 

Colored  . 

}  Biaxial  .... 

.     16 

Pleochroic  .  . 

(  Uniaxial  .  .  . 

.     20 

vi 


THE  MINERAL  IS  OPAQUE 


[Magnetite 
Black  l>y  incident  light  ...................................................................  {Graphite 

(llmenite 

[  Red  .........................  Hematite 

Transparent  on  thin  edges  ....................................  <  Brown  .......................  Chromite 

I  Greenish-brown  ...............  Picotite 

Pyrite  occurs  in  crystals  and  irregular  grains  as  a  primary  accessory  in  igneous  rocks,  and  abun- 
dantly  in  mt'tamorphic  rocks  and  sediments.  It  is  non-magnetic,  and  insoluble  in  HC1.  By  incident 
light  the  color  is  lighter  than  that  of  pyrrhotite. 

1'ijrrhotite  occurs  rarely  in  igneous  rocks,  abundantly  with  certain  ore  deposits.  Pyrrhotite  has 
a  red-bronze  to  yellow-bronze  color;  p  y  r  i  t  e  is  brass-yellow.  Pyrrhotite  is  magnetic. 

Magnetite  occurs  in  the  form  of  octahedrons,  cubes,  or  irregular  grains  as  a  common  accessory  in 
all  kinds  of  igneous  rocks,  most  abundantly  in  those  that  are  basic.  It  also  occurs  in  metamorphic 
rocks  and  sediments.  As  a  secondary  mineral  it  is  found  in  igneous  rocks,  in  some  cases  in  dustlike 
or  dendritic  forms.  It  is  magnetic. 

Graphite  occurs  in  irregular  flakes  or  scalelike  aggregates,  rarely  in  hexagonal  plates,  as  a  con- 
stituent of  metamorphic  rocks,  schists,  quartzites,  marbles,  rarely  in  pegmatites  or  other  igneous 
rocks.  May  not  be  separable  from  magnetite  when  the  latter  occurs  in  irregular  grains. 

Ilmenite  is  an  accessory  mineral  in  igneous  rocks,  especially  in  diorites,  gabbros,  diabases,  etc., 
also  in  metamorphic  rocks.  It  occurs  in  hexagonal  plates  or  grains,  which  in  many  cases  are  altered 
over  the  entire  surface,  or  along  lines  intersecting  at  60°,  to  a  white  decomposition  product,  usually 
tit  unite,  called  leucoxene.  Titaniferou's  magnetite  also  may  alter  to  leucoxene.  Ilmenite 
rarely  shows  deep-brown  thin  edges. 

In  some  cases  magnetite,  graphite,  and  ilmenite  may  not  lie  separable  under  the  microscope. 

I 

Hematite  is  found  in  rocks  of  all  kinds,  either  as  small  hexagonal  crystals,  rare  in  igneous  rocks, 
as  pseudomorphs  after  magnetite,'  as  rims  around  magnetite,  as  an  alteration  product  from  various 
ferromagnesian  minerals,  and  as  stains  in  cleavage  cracks.  It  occurs  in  immense  deposits  among 
sediments.  Magnetite  is  black  by  incident  light,  hematite  is  red.  L  i  m  o  n  i  t  e  is  usually 
yellow  although  it  may  be  red,  in  which  case  it  may  be  confused  with  hematite.  In  such  cases  it  is 
customary  to  speak  of  the  material  as  red-,  brown-,  or  yellow  iron  oxide,  and  let  it  go  at  that. 

Chromite  is  a  mineral  of  peridotites  and  serpentines.  It  is  black  by  incident  light,  sometimes 
brownish  black  on  thin  edges.  In  many  cases  it  may  not  be  possible  to  separate  chromite  from  magne- 
tite except  by  the  reaction  for  chromium. 

Picotite,  the  chrome  spinel,  occurs  as  an  accessory  in  peridotites  and  other  basic  igneous  rocks, 
in  serpentines  derived  from  peridotites,  and  rarely  as  crystals  in  basalts.  It  is  greenish  brown  to 
yellowish  brown  on  thin  edges,  and  is  isotropic. 

1 


C  THE    MICROSCOPICAL    DETERMINATION 


THE  MINERAL  IS  ISOTROPIC 


Occur  as  crystals 


Colorless 


Refractive  index  is  less  than   Canada 
balsam  (1.537) 


Fill  cavities  or  are  amorphous 


Fluorite n  =  1.433 

Sodalite 1 . 483 

Noselite 1.490 

Hauynite. ...  1.503 

Leucite 1.508 

/Opal 1.443 

\Glass ±1.490 


Refractive  index  is  greater  than  Canada 
balsam 


Cavity  or  interspace   filling  or  at- /Fluorite 1.433 

tached  crystals  \Analcite 1 .488 

Form    octahedrons,    usually    show /Spinel 1.716 

quadratic  sections  \Periclase.  .  .  .  1.736 


Usually    in    polygonal    sections    or/Grossular. . .  . 
rounded  grains  \Spessartite. . . 


1.750 
1.811 


Refractive  index 
balsam 


Colored 


is  less  than  Canada! 


(Occur  as  crystals 


[Fills 


s  cavities  or  is  amorphous 


Fluorite .... 
Sodalite .... 
Noselite. . . . 
Hauynite. . . 

Leucite 

Glass .  . 


433 

483 

490 

503 

1.508 

1.490 


Refractive  index  is  greater  than  Canada 
balsam 


Form    octahedrons, 
quadratic  sections 


usually 


Spinel 

Periclase .  . 
Hercynite . 
Picotite .  . . 
show/Gahnite. . . 
Pleonaste. . 
Pyrrhite. . . 
Beckelite.  . 
Perofskite . 


1.718 
1.736 
1.749 
=  1.7 
1.765 
High 
High 
High 
2.38 


Rounded,  quadratic,  hexagonal,  oc- 
tahedral, etc.,  or  in  irregular  grains. 
Usually  with  strong  fractures 


Grossularite., 

Pyrope 

Almandite.  .  . 
Spessartite. . 
Uwarowite. . . 
Melanite .  . 


.744 
744 
.810 


1.811 


.838 
.856 


Fluorite,  when  colored,  is  readily  separated  from  all  other  minerals  of  low  index.  The  color,  in 
many  cases,  is  irregularly  distributed.  Cleavage  (111)  is  perfect.  The  mineral  rarely  forms  crystals, 
but  generally  occurs  as  cavity  or  interspace  filling.  It  never  shows  anomalous  double  refraction. 
S  o  d  a  J  i  t  e  and  analcite,  with  which  it  may  be  confused  when  colorless,  differ  in  cleavage, 
have  anomalous  double  refraction  in  many  cases,  and  occur  in  the  alkalic  rocks,  while  fluorite  is 
especially  confined  to  acid  granites  and  pegmatites,  frequently  associated  with  topaz,  tourmaline,  and 
tin,  where  it  was  formed  by  pneumatolytic  action. 

Sodalite,  a  mineral  of  the  nephelite-  and  other  alkalic  syenites,  has  a  fair  (110)  cleavage  while 
analcite  has  fair  (100)  cleavage.  The  two  cannot  be  separated  by  optical  means,  but  chemically 
the  presence  of  chlorine  indicates  sodalite  (Manual,  p.  563).  Optical  anomalies  are  common  in  both. 

Noselite,  hauynite,  and  leucite  always  occur  as  crystals  and  are  confined  to  igneous  rocks,  where 
they  are  primary  minerals.  Leucite  occurs  in  six-  or  eight-sided  to  rounded  grains,  in  many  cases 
with  characteristic  radial  or  tangential  inclusions  in  regular  zones.  Small  leucites  are  generally  iso- 
tropic,  but  larger  grains  are  almost  invariably  polysynthetically  twinned  in  a  pattern  resembling 
microcline,  though  of  much  lower  birefringence  (Manual,  p.  510).  Noselite  and  hauynite  cannot  be 
separated  under  the  microscope  by  optical  tests,  but  may  be  separated  microchemically  (Manual, 
p.  563). 


OF    RorK-FoRMlM!    MlNKRALS   AND    ROCKS 


Analrite  is  found  filling  veins  and  cavil ic>  in  lui.-alis.  diabase*,  and  other  basic  rooks,  and  as 
interspace  tilling  in  certain  liasaltic  rocks,  where  it  has  been  considered  a  primary  mineral.  It 
can  U-  .-eparated  from  sodalite  only  by  microchemical  means  (Manual,  p.  564).  Anomalous 

doulilr  refraction   is  common. 

Opal  has  no  cleavage  and  may  show  anomalous  double  refraction.     It  occurs  in  cavities  and 

veins  in  acid  igneous  rocks  as  a  deposit  from  magmatic  waters,  and  as  nodules  in  limestones,  -hales, 
sandstone-,  -ilicitied  wood.  etc. 

(flux*  has  no  cleavage.  It  does  not  occur  in  the  plutonie  rocks  except  as  inclusions  in  feldspars, 
etc..  luit  is  common  in  the  acid  extrusives.  Its  uniform  distribution  through  the  slide  separates  it 
from  opal.  Its  lack  of  cleavage  separates  it  from  sodalite  and  a  n  a  1  c  i  t  e  . 

Spinel  occurs  as  an  accessory  in  peridotites  and  other  basic  igneous  rocks,  in  serpentines  derived 
from  peridotites,  in  granular  limestones,  gneisses,  etc.  It  is  pale  red,  green,  or  blue,  and  has  poor 
(111)  cleavage. 

1'iricliixf  is  gray  to  yellow,  and  has  good  (100)  cleavage.  Both  periclase  and  spinel  occur  in 
quadratic  (111)  crystals. 

is  dark  green,  picotite  is  yellow-brown  to  greenish  brown,  gahniie  is  greenish  black, 
is  green,  pyrrhite  is  orange-yellow  to  red,  beckelite  is  pale  yellow,  and  perofxkite  is  grayish 
white,  brownish  to  red-brown,  rarely  greenish  gray.  Hercynite,  gahnite,  and  pleonaste  are  separable 
only  chemically,  the  others  by  color. 

The  garnets  all  show  lack  of  cleavage,  high  relief,  and  isotropism. 

UniKxtihir  is  colorless  to  pale  yellow.  It  may  show  anomalous  double  refraction.  Zonal  struc- 
ture is  common.  Occurs  in  metamorphic  calcareous  rocks,  as  a  contact  mineral,  or  in  crystalline 
schists. 

Spessarlite  is  blood-red,  yellowish  red,  or  red-brown  to  colorless.  Anomalous  double  refraction 
i-  common.  It  occurs  in  granites,  a  rhyolite  (C'olorado),  and  quartzites. 

Pyrope  is  red  to  blood-red.  Kelyphite  rims  are  common.  The  mineral  occurs  only  in  peridotites, 
dunites,  and  their  derivatives. 

Almandite  is  red  to  red-brown,  zonal  structure  is  common,  but  anomalous  double  refraction  does 
not  occur.  The  mineral  is  found  in  igneous  rocks  which  have  been  dynamo-metamorphosed,  and  in 
mica-schists  and  other  metamorphic  rocks. 

Uwarounte  is  deep  green  and  usually  shows  anomalous  double  refraction.  It  is  confined  to 
chromium-rich  serpentines,  granular  limestones,  and  dolomites.  Alterations  are  unknown. 

Melanite  occurs  in  various  tones  of  dark  brown,  less  commonly  in  green.  Anomalies  are  hardly 
observable  owing  to  the  deep  color.  Zonal  structure  is  common,  but  alterations  are  wanting.  The 
mineral  is  found  in  various  igneous  rocks,  especially  phonolites,  nephelinites,  leucitites,  tephrites,  etc., 
also  in  contact  metamorphosed  rocks. 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The    Mineral     is     AN 
The  Mineral  is  NEGATIVE  ( 

SOTROPIC,      COLORI 

-)•                                   1 
eate  in  Index  of  Refrac 

.ESS,     UNIAXIAL. 
he  Mineral  it  POSITIVE  (  +  ). 

' 

- 

Sooo  w             o              w              o              m  §          o             w 
OO^x   r-                 r,                 to                  to                 in   5             in                 v 

CNCN—  ~    •»                       M                        ••                         ^                         „  J                   _                       „ 

In 

cress'g 
BIREFH. 

9  •        '•         1  M             o             ui             o            ""=022 
v               m           J  10               «s               to               r~              r-ooffiOw 

-              „          1  „               _               -               -              -_-NN 

j 

VerrH, 

Hgh. 

Medium. 

Not  Marked.  u             LOW. 

1 

LOW.'           °  Not  Marked. 

Medium. 

High. 

Verj  H. 

Euco 
Apatite 

lite.  . 
«.!—  .f 

ephelite 

Tridyr 

Leucite 
nite.* 

-  Eudii 

lite. 

Vesuv 

anite 
-»-Corund 

Melilit 

Quartz. 

-  Quartz 

Dipyr.  - 

010 

• 

Hydro 

1C 

:>helite 

Alunite. 
rucite. 

B 

•    Cancr 

nite. 

Vleionite. 

.040 

Phlo 

;opjte.    - 

.045 

050 

Zircon 

-  Am 

tase. 

075 

080 

.090 
.095 
.100 
.120 
140 

Cassiteri 

e.  - 

Mag 

c       Cal( 

ite. 

.160 
.200 
.250 

erite. 

OF    RoCK-FoUMIX<i    MlSKUALS   AND    ROCKS 


The  Mineral  is  Negative 

•  >lite  occurs  in  various  nephelite-syenites. 
It  lias  very  weak  (O>E)  plooohroism  or  none. 
Cleavage  (0001)  is  distinct.  Anomalous  21  ta 
50°.  K  u  (1  i  a  1  y  t  e  is  optically  positive  and 
has  negative  elongation.  T  o  p  a  z  is  biaxial. 
Apatite  has  poor  cleavage,  long  crystals 
shu\\  parting,  and  elongation  is  negative. 
M  e  1  i  !  i  i  e  has  characteristic  abr»ormal^erlin 
blue  interference  color  and  basal  cleavago^J 

Apatite  has  characteristic  basal  parting  in 
long  prisms.  It  is  easily  soluble  in  HjSO4  and 
the  solution  gives  a  itfow  precipitate  with 
ammonium  molybdatc  (Manual,  p..  565).  Apa- 
tite is  a  very  common  accessory  in  the  form  of 
small  prisms  in  most  igneous  rocks.  In  large  crys- 
tals it  occurs  in  pegmatites,  some  lamprophyres, 
It  is  also  found  in  crystalline  schists,  lime- 
si  ones, argillites, etc.  Sillimanite  has  higher 
double  refraction  and  positive  elongation. 

'Hi'  is  a  feldspathoid  and  does  not 
occur  in  qovte-beftring  rocks.  It  usually  shows 
abnormal  Berlin  blue  interference  colors.  The 
(001)  and  (110)  cleavages  are  poor;  only  the 
basal  cleavage  is  generally  seen  in  thin  sections, 
and  this  occurs  as  a  single  cleft  along  the  middle 
of  the  lath-shaped  section.  Peg  structure,  due 
to  inclusions  growing  inward  from  basal  sections, 
is  characteristic.  It  gelatinizes  easily  with  HC1 

•i  ual,  p.  564).  Vesuvianite  and 
•/.  o  i  s  i  t  e  ,  both  of  which  may  give  the  abnor- 
mal blue  interference  color,  are  insoluble  in 
acids.  Vesuvianite  has  higher  relief,  and  usually 
occurs  as  a  contact  mineral  in  limestone.  Zoisite 
is  biaxial  and  occurs  as  a  secondary  mineral. 

Nephelite  occurs  in  grains  in  soda-rich 
plutonic  rocks,  and  in  grains  and  quadratic 
sections  in  extrusives,  but  it  is  never  found  in 
the  same  rock  with  primary  quartz.  It  has 
rather  distinct  (0001),  (1010)  cleavages.  Anoma- 
lous biaxial  character  with  small  optic  angle 
may  occur.  It  gelatinizes  easily  with  HC1 
(Manual,  p.  564).  Nephelite  resembles  q  u  a  r  t  z 
in  appearance,  but  is  negative.  Cordierite 
is  biaxial.  Scapolites  occur  in  metamorphic 
rocks,  gneisses,  crystalline  schists,  granular  lime- 
stones, etc.,  but  are  rarely  primary  in  igneous  rocks. 


The  Mineral  is  Positive 

Leucite  is  isometric  at  433°  C.,  but  below 
that  temj>erature  is  doubly  refracting  (Manual, 
p.  510).  Characteristic  radial  or  tangential 
inclusions,  twinning,  and  crystal  form  separate 
it  from  all  other  minerals.  It  is  found  only  in 
igneous  rocks  which  are  high  in  potash  and  low 
in  silica,  never  in  sediments  or  as  a  metamorphic 
mineral.  It  is  fairly  common  among  extrusives 
but  is  very  rare  among  plutonites. 

Tridymite  is  characterized  by  low  refractive 
indices  and  by  its  occurrence  in  tabular, 
hexagonal,  or  rounded  crystals,  or  in  rosette-like 
aggregates,  or  in  overlapping  plates  so  small 
that  edges  appear  like  rectangular  cleavage  lines. 
Anomalous  optic  angle,  2E,  may  be  as  high  as  70°. 
The  mineral  occurs  in  cavities  in  silicic  extrusive 
rocks.  It  resembles  no  other  minerals. 

Eudialyte  is  optically  positive  and  has  nega- 
tive elongation,  while  e  u  c  o  1  i  t  e  is  negative 
and  has  positive  elongation.  Anomalous  2E 
may  be  as  high  as  50°.  It  is  commonly  associated 
with  nephelite.  See  under  eucolite. 

Quartz  usually  shows  no  cleavage  It  may 
be  separated  from  nephelite  by  the  cleav- 
age and  negative  character  of  the  latter.  Cor- 
dierite is  biaxial  and  negative,  with  2V  from 
40°  to  84°,  but  quartz  may  show  an  anomalous 
optic  angle,  2E,  in  some  cases  as  high  as  25°. 
When  cordierite  is  treated  with  HF  it  gives 
characteristic  prismatic  crystals  of  magnesium 
fluosilicate.  Scapolites  are  negative,  show 
cleavage,  and  have  higher  double  refraction. 

Hydronephelile,  a  rare  mineral  in  igneous 
rocks,  has  a  poor  (lOlO)  cleavage  and  occurs  in 
rodlike,  leafy,  or  granular  aggregates.  It  is 
separated  from  nephelite  by  its  positive 
character  and  lower  indices ;  from  quartz  by 
lower  indices,  from  thomsonite  by  its 
lower  birefringence.  It  is  soluble  in  HC1  with 
the  formation  of  jelly. 

Alunite  occurs  in  veins  in  certain  extrusive 
rocks  and  is  formed  by  the  action  of  SOj  upon 
them.  It  has  a  good  (0001)  cleavage,  which  dis- 
tinguishes it  from  quartz,  as  does  also  its 
higher  birefringence. 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The  Mineral  is  Negative 

Vesuvianite  has  poor  (110),  (100)  cleavages. 
It  usually  occurs  as  a  contact  mineral  derived 
from  limestone,  but  has  also  been  found  in  ancient 
ejected  blocks  among  the  dolomite  masses  of 
Vesuvius  and  Monte  Somma.  In  some  cases  it 
shows  abnormal  Berlin  blue  interference  colors, 
or  biaxial  character.  It  is  insoluble  in  acids 
unless  first  fused.  Z  o  i  s  i  t  e  has  better  cleav- 
age and  a  different  mode  of  occurrence. 

Corundum  occurs  as  a  primary  mineral  in 
alumina-rich  igneous  rocks,  both  acid  and  basic, 
such  as  pegmatites,  syenites,  anorthosites,  and 
dunites.  It  is  rare  as  a  contact  mineral,  but 
occurs  in  granular  limestones  and  dolomites, 
gneisses,  mica-schists,  etc.  The  pleochroisrn, 
O  =  blue,  red,  E  =  sea-green,  yellow,  or  greenish 
yellow,  is  seen  only  in  deeply  colored  specimens. 
It  has  poor  parting  (1011),  (0001).  The  high 
relief  separates  it  from  similar  minerals  except 
vesuvianite  from  which  it  is  separated 
by  its  hardness,  higher  double  refraction,  and 
by  chemical  means. 

Dipyr  (mizzonite),  wernerite  (common  scapo- 
lite),  and  meionite  are  scapolites,  and  may  be 
considered  as  isomorphous  mixtures  of  the  two 
molecules  marialite  (Ma)  (SNa^O-SAWVlSSKV 
2NaCl)  and  meionite  (Me)  (4CaO-Al2O3-6Si02). 
eo  to  MajMei  is  called  dipyr  (mizzonite), 
to  MaiMe2  wernerite,  and  MaiMe2  to 
oo  meionite.  Where  cleavage  is  not 
shown,  scapolites  resemble  quartz,  but  their 
birefringence  is  greater  and  they  are  negative. 
The  mode  of  occurrence  and  birefringence  sepa- 
rate them  from  nephelite,  as  does  also  the 
cleavage,  which  appears  right-angled  in  sections 
giving  an  interference  figure.  They  occur  in 
metamorphic  rocks,  gneisses,  schists,  in  contact 
metamorphosed  limestones,  and  as  secondary 
minerals  in  calcium-rich  basic  rocks.  Optical 
anomalies  showing  the  opening  of  the  inter- 
ference cross  are  rare.  They  readily  alter  to 
mica,  etc. 

Cancrinite,  a  secondary  mineral  after  nephe- 
lite, by  some  considered  in  part  primary,  resembles 
muscovite  in  its  high  interference  colors, 
but  its  indices  of  refraction  are  less  than  Canada 
balsam.  No  other  common  mineral  has  such 
high  colors  and  low  indices  except  t  h  o  m  - 
s  o  n  i  t  e  and  hydrargillite,  but  both  of 
these  are  biaxial  and  positive.  For  chemical 
separation  of  cancrinite  from  nephelite  and 
hydronephelite  see  elsewhere  (Manual, 
pp.  564-65). 


The  Mineral  is  Positive 

Brucite  is  a  secondary  mineral  found  in 
serpentines  and  magnesian  limestones.  It  usually 
occurs  in  foliated  or  fibrous  masses,  sometimes 
spherical,  or  in  plates.  Anomalous  biaxial  char- 
acter is  not  rare.  Muscovite  and  talc 
are  negative  and  have  positive  elongation. 
Hydromagnesite  has  lower  birefringence 
and  effervesces  with  HC1  while  brucite  is  soluble 
without  effervescence.  Gypsum  is  very  simi- 
lar in  appearance,  but  its  indices  are  lower  than 
balsam,  and  it  has  inclined  extinction. 

Zircon  has  weak,  seldom  noticeable  pleo- 
chroism.  It  occurs  in  small  characteristic  crys- 
tals which  are  shorter  and  stouter  than  those  of 
apatite,  and  which  have  brilliant  interfer- 
ence colors.  In  larger  grains,  the  interference 
colors  are  pale  and  the  mineral  is  brownish. 
Zircon  is  especially  common  in  acidic  and  in  sodic 
igneous  rocks,  but  is  also  found  in  schists  and 
gneisses,  and  as  a  residual  mineral  in  the  decom- 
position products  of  igneous  rocks. 

Cassiterite  may  be  pleochroic  in  weak  brown- 
ish tones.  Cleavage  (110)  is  poor,  (100)  dis- 
tinct. Geniculated  twins  are  common.  It  occurs 
as  a  pneumatolytic  mineral  in  acid  dikes  and 
quartz  veins,  and  as  a  rare  primary  mineral  in 
some  igneous  rocks.  R  u  t  i  1  e  has  better  cleav- 
age and  is  not  so  brown,  anatase  is  negative, 
brookite  is  biaxial,  and  perofskite  is 
isotropic. 


Or    HIM  K-l-'nllMIM.     MlNKKM.x    AMI    KiM'KS 


The  Mineral  is  Negative 

Mi  iiinitr.     Sec  under  ilipyr.  above. 

1'hltHjitpitf  is  generally  at  least  faintly  colored 
with  weak  yellowish  pleochroism.  The  "bird's- 
cyc  maple"  appearance  separates  it  from  all 
minerals  except  other  micas  and  talc.  Its 
imiaxial  interference  figure  separates  it  from 
all  luit  talc  and  I)  i  o  t  i  t  e  .  The  former  occur- 
only  in  basic  rocks  and  is  perfectly  colorless  or 
faintly  green,  the  latter  is  usually  strongly  plco- 
chroic.  Bleached  biotite  may  l>e  color- 
less  and  impossible  to  separate  from  phlogopite. 


occurs  in  pyramids  and  tablets, 
and  is  found  in  igneous  rocks  in  some  granitc- 
pegmatitos.  It  usually  has  pleochroism,  0  = 
deep  blue  or  orange-yellow,  E  =  light  blur  or 
light  yellow,  but  it  may  lw  very  weak  so  that  tin- 
mineral  appears  eolorlr—  in  thin  sections.  Color- 
less or  yellow  portions  are  usually  normal,  while 
blue  portions  show  anomalous  opening  of  the 
interference  cross  and  do  not  fully  extinguish. 
Perofskite  differs  in  form,  and  the  anoma- 
lou-  interference  colors  are  lower  than  those  in 
anatase. 

Calotte,  dolomite,  and  magnesite  cannot  be 
separated  under  the  microscope,  but  may  be  by 
chemical  means  (Manual,  p.  565).  Aragon- 
it  e  is  biaxial  with  2V  =18°,  and  it  differs  in 
certain  chemical  reactions  (Manual,  p.  568). 
B  r  u  c  i  t  e  differs  chemically  (Manual,  p.  567), 
and  has  much  lower  double  refraction.  Calcite 
is  a  common  alteration  mineral  in  all  kinds  of 
igneous  rocks,  and  is  said  to  be  primary  in  some 
granites.  Both  calcite  and  dolonu'te  occur  as 
vein  minerals,  and  in  widespread  and  thick 
st  rata.  Magnesite  occurs  as  a  secondary  mineral 
derived  from  the  alteration  of  magnesia-bearing 
minerals.  It  also  occurs  in  talc-schists,  serpen- 
tines, etc.,  often  as  veins. 

'•ite  has  higher  indices  of  refraction  than 
the  three  preceding  carbonates,  and  is  usually 
somewhat  yellowish  or  brownish.  It  is  a  com- 
mon mineral  of  ore  veins  and  of  limestones.  It  is 
also  found  in  gneisses,  slates,  shales,  gray-wackes, 
etc. 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The     Mineral     it 
The  Mineral  ii  NEGATIVE  (- 

\NISOTROPIC,      COLC 
-).                                   Tl 
.ase  in  Index  of  Refract 

RLESS.     BIAXIAL, 
ic  Mineral  ii  POSITIVE  (+). 

ooooin             o             in              o             in  E         o              in 
m  o  o>  oo  h.              i*              to               (o              ing          in              ^ 

lit 

crcasg 
BIREFH. 

I 

in              o           Em               o             m              o             inoooo 

-a 

n'ri.  -                  -                 -J 

VerjH. 

High. 

Medium. 

Not  Marked,  o            LOW. 

LOW. 

0  Not  Marked. 

Medium. 

High. 

VertH. 

i 

Inorthoc 
Caolinj- 
igoclase, 
nite.  * 

las 

_ 

^r' 

-  M 

.TnrH 

anidine. 
rthoclas 
-  Stilbi 
icrocline 

Epistilbi 

>aumonti 

• 

.005 

Quartz. 
.010 

.015 
.020 

O 
Bytow 

e, 

'.  — 

Hel 

landite.  ' 

•    Andesinc. 
—  Labradorite. 

/  Bro 

izite. 
—  Clino 

oisite. 
—  -^ 
elite.  - 

r   n< 

m.  —  ^ 
dony." 
ntine.  • 

/ 

Enstatite 

r 

Dist 
Hyp 

lene.  -• 
ersthene.  • 

• 
jllastonit 

Vnorthite 

' 

— 

nl 

igorite 

—  I 

tc. 

Chalce 
Serpe 

^  Sp 

odumene. 

_  ± 

—    Sillimanite. 
.  Anthophyllite.i 

Tremo] 
Actinoi 

lie.  — 
ite. 

030 

Diopsi 

ie.   _ 

Diallage. 

Epido 

te. 

035 

__  For 

sterite. 

Mus 
Par: 

:ovite.   i 
goni'te.  1 

^^    ] 

'ectolite. 

Dat 

Jlite.  _ 

___  I 

>h 

-CF 

ogopit 
idolite 

e. 

_  Anhj 

drite. 

M 

.085 

.090 
.095 
.100 
.120 

Titanite.- 

Aragonit 

.160 
.200 
,250 

Broo 

lite.  ~ 

OP  ROCK-FORMINO  MINERALS  ANT>  ROCKS 


The  Mineral  is  Negative 

Kaolin  usually  appears  as  a  flour-like,  white, 
opaque  alteration  protlm-t  of  feldspar.  It  may 
l>e  stained  yellow  or  red  l>y  iron  oxides.  When 
crystallized  it  occurs  in  the  form  of  leaves  and 
scales  with  an  extinction  angle  of  13°,  and  may 
be  mistaken  for  sericite,  although  its  bire- 
fringence is  lower.  Much  so-called  kaolin  is 
colloidal  aluminium  silicate,  and  not  kaolin. 
Muscovite  (sericite)  and  h  y  d  r  a  r  g  i  1 1  i  te 
have  higher  birefringence^. 

.1  nurthodase  (soda-microcline),  orthoclase,  san- 
.  oligoclase,  and  microdine  are  feldspars. 
Social  methods  for  their  separation  are  given 
on  pages  30-34.  Anorthocla.se,  orthoclase,  sani- 
dine,  and  mieroclino  have  indices  less  than 
(  anada  balsam.  Anorthoclase  has  2V  =  43°-53°, 
orthoclase  2V  =  69°43',  2E=  121°6',  sanidine  2V  = 
small  to  0°,  microcline  2V  =  83°.  It  may  be 
impossible  to  separate  anorthoclase  and  ortho- 
clase under  the  microscope.  Microcline  is  sepa- 
rated by  the  "grating"  or  plaid  structure,  due 
to  the  combination  of  polysynthetic  albite  and 
pericline  twinning.  Oligoclase  usually  has  poly- 
synthetic  twinning  with  characteristic  extinction 
angles,  and  refractive  indices  very  near  Canada 
balsam,  in  some  sections  greater  in  one  direction 
and  less  in  the  other.  All  of  these  feldspars 
are  separated  from  albite  by  the  positive  char- 
acter of  the  latter,  or  by  its  extinction  angle. 
<  >rt  hoclase  and  microcline  may  contain  inter- 
grown  lamellae  of  albite  (less  commonly  oligo- 
clase), and  are  then  called  microperthite 
and  microcline-microperthite.  In 
anti-perthite,  orthoclase  forms  lamellae 
in  oligoclase  or  andesine. 

Stilbile,  one  of  the  zeolites,  occurs  in  rods, 
leaves,  and  sheaflike  or  radiating  groups,  in 
cavities  in  basalts,  and  less  commonly  in  granites. 
The  extinction  angle  c:o  is  about  8",  and  2V  is 
about  33°.  It  is  decomposed  by  HC1  without 
gelatinization. 

Cordieriie,  orthorhombic,  2V  from  40°  to  84°, 
2E  from  63°  to  150°,  occurs  in  gneisses  and  various 
schiste,  rarely  as  a  primary  mineral  in  granites, 
andeaites,  etc.  As  a  contact  mineral  it  is  found 
at  the  contact  of  acid  igneous  rocks  with  shales 
and  slates.  When  treated  with  HF  it  gives 
characteristic  prismatic  crystals  of  magnesium 
fluosilicate.  Pleochroic  halos  are  occasionally 
seen  in  sections  parallel  to  the  c  axis.  Trillings 
and  polysynthetic  twins  occur.  Q  u  a  r  t  z  is  uni- 
axial  and  positive,  albite  is  positive  and  has 
lower  indices  of  refraction,  nephelite  is  uni- 
axial. 


The  Mineral  is  Positive 

Heulanditf,  a  zeolite,  occurs  in  leaves,  plates, 
or  rosettes,  in  basaltic  rocks,  rarely  in  gneiss. 
Its  biaxial  character  (2E  =  0°-55°),  low  indices 
and  birefringence,  and  its  habit  separate  it  from 
other  minerals. 

Andesine  and  labradorite  are  plagioclase  feld- 
spars. They  usually  show  polysynthetic  twin- 
ning with  characteristic  angles.  See  pages  31-34. 

Clinozoisite  is  an  iron-poor  or  iron-free  epidote 
with  the  composition  of  zoisite.  It  is  colorless 
or  reddish  with  weak  or  no  pleochroism,  extinc- 
tion angle  of  3°,  and  a  large  optic  angle  (2V  = 
80°-90°).  It  occurs  in  prisms  or  rods  elongated 
on  l>.  and  in  grains.  Abnormal  Berlin  blue 
interference  colors  are  common,  as  in  zoisite, 
but  this  has  parallel  extinction  and  smaller  optic 
angle  (2V  =  0°-60°).  It  may  be  impossible  to 
separate  the  usual  grains  found  in  igneous  rocks 
from  zoisite.  P  i  s  t  a  c  i  t  e  has  higher  double 
refraction. 

Topaz  occurs  as  a  pneumatolytic  mineral  in 
granite  dikes,  granites,  rhyolites,  and  cassiterite- 
pegmatites,  either  in  cavities  or  scattered  through 
the  rock.  It  is  also  found  in  the  adjacent  schists 
and  gneisses.  Cleavage  is  basal,  but  may  not 
show  in  thin  sections.  Quartz  has  lower 
relief  and  is  uniaxial.  Andalusite  is  nega- 
tive and  usually  has  different  mode  of  occurrence. 
Apatite  is  uniaxial  and  negative,  while 
topaz  has  a  large  optic  angle  (2E  =  71°-129°). 
Vesuvianite  is  usually  slightly  pleochroic 
and  is  uniaxial.  Disthene  is  negative,  2V  = 
82°,  and  elongation  is  positive.  Corundum 
is  uniaxial  and  negative. 

Enslaiite  has  the  usual  pyroxene  cleavage, 
parallel  extinction  (see  note  under  hypersthene), 
2E  =  135°,  and  is  non-pleochroic.  Mono- 
clinic  pyroxenes  have  higher  birefring- 
ence and  inclined  extinction  in  sections  at  right 
angles  to  the  principal  optic  sections.  In  basal 
sections  (which  have  sharp  cleavage  lines  at 
angles  of  approximately  90°  with  each  other) 
monoclinic  pyroxenes  show  the  emergence  of  an 
axis  while  orthorhombic  pyroxenes  show  the 
emergence  of  a  bisectrix.  Bronzite  is 
slightly  pleochroic,  hypersthene  is  pleo- 
chroic and  negative. 

[ironzite  is  essentially  like  enstatite  but  it  is 
slightly  pleochroic  in  green  and  pink  tones. 
2E=  =t  106°.  Hypersthene  has  similar  but 
stronger  pleochroism  and  is  negative.  For  sepa- 
ration from  monoclinic  pyroxenes,  see  under 
enstatite  above. 


10 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The  Mineral  is  Negative 

Epistilbite,  a  zeolite  with  inclined  extinction, 
c:c=— 9°,  occurs  in  basaltic  rocks.  It  is  im- 
perfectly soluble  in  concentrated  HC1  without 
gelatinization.  May  not  be  possible  to  separate 
from  s  t  i  1  b  i  t  e  ,  although  the  birefringence  of 
the  latter  is  lower. 

Andalusite  has  characteristic  though  fre- 
quently faint  pleochroism,  a  =  rose,  b  =  c  =  color- 
less  to  light  green,  resembling  that  seen  in  hyper- 
sthene.  Hypersthene,  however,  has  posi- 
tive elongation,  while  andalusite  has  negative. 
Cleavage  also  differs,  but  the  good  (110)  and 
less  distinct  (100)  cleavage  of  andalusite  may  not 
be  seen  in  thin  sections,  where  the  mineral  often 
appears  in  irregular  grains.  In  the  schists  the 
mineral  usually  occurs  in  characteristic  irregular 
oval  grains  associated  with  grains  of  magnetite. 
In  chiastolite  the  inclusions  are  found  in 
regular  arrangement,  in  the  forms  of  rhombs, 
crosses,  etc.,  in  cross-sections  and  parallel  to  the 
long  axes  of  prisms,  and  the  material  is  often 
altered  to  a  mica-like  mineral.  The  higher 
relief  separates  andalusite  from  cordierite. 
Andalusite  is  found  in  a  few  granites,  but  is 
essentially  a  mineral  of  slates,  schists,  and  gneisses. 
Chiastolite  is  a  contact  mineral  in  argillites  near 
granitic  intrusions. 

Antigorite,  the  massive,  lamellar  serpentine, 
differs  from  common  serpentine,  which 
is  fibrous,  in  being  negative,  and  in  its  habit. 
Pennine,  when  optically  negative,  is  sepa- 
rated by  its  optical  character,  when  positive,  by 
a  chemical  test  for  A1203.  Pennine  also  has 
lower  birefringence,  usually  abnormal  interference 
colors,  and  pleochroism.  Serpentines  are  always 
secondary  and  occur  as  an  alteration  product  of 
olivine,  less  commonly  of  pyroxene  or  amphibole, 
and  possibly  also  of  other  ferromagnesian  min- 
erals. 

Disthene  (cyanite)  does  not  occur  in  igneous 
rocks,  but  chiefly  in  muscovite  or  paragonite 
schists,  gneisses,  eclogites,  etc.,  often  associated 
with  garnets  or  corundum.  The  color  is  faint 
blue  in  thin  sections,  in  some  cases  almost  color- 
less. Cleavages,  (100)  perfect,  (010)  distinct, 
making  an  angle  of  74°,  are  very  characteristic, 
although  they  do  not  show  in  all  sections.  Orien- 
tation, a  is  nearly  at  right  angles  to  (100),  c  is 
inclined  30°  on  (100)  to  the  edge  (100):  (010). 
S  i  1 1  i  m  a  n  i  t  e  and  andalusite  are 
orthorhombic  and  have  different  cleavages, 
topaz  has  basal  cleavage  only,  z  o  i  s  i  t  e 
usually  has  abnormal  interference  colors  and 
occurs  in  grains. 


The  Mineral  is  Positive 

Zoisite,  orthorhombic,  is  a  mineral  of  the 
crystalline  schist  formations,  produced  by  the 
dynamo-metamorphism  of  igneous  rocks  contain- 
ing basic  plagioclase.  It  also  occurs  in  pegma- 
tite dikes.  Abnormal  Berlin  blue  interference 
colors  are  common.  Cleavage  (010)  good,  (100) 
distinct.  Clinozoisite  has  an  extinction 
angle  of  3°  and  an  optic  axial  angle  of  2V  =  80°-90°, 
while  zoisite  has  an  angle  of  0°-60°.  M  e  1  i  - 
lite  gelatinizes  with  acids,  occurs  only  in 
quartz-free  rocks,  and  has  a  characteristic  habit. 
Vesuvianite  has  poorer  cleavage  and  higher 
relief. 

Gypsum  is  a  mineral  of  the  stratified  rocks 
and  occurs  in  connection  with  limestones  and 
other  rocks,  but  is  rarely  found  in  the  crystalline 
schists.  It  resembles  muscovite  but  the 
birefringence  is  lower,  its  extinction  angle  (c:c  = 
-52°  to  -53°)  is  higher  (muscovite,  c:c  =  0°-2°), 
and  it  does  not  show  the  "bird's-eye  maple" 
effect.  Anhydrite  is  not  so  shredded  and 
has  higher  double  refraction  as  well  as  parallel 
extinction. 

Albite,  a  plagioclase,  usually  shows  poly- 
synthetic  twinning  with  characteristic  extinc- 
tion angles.  When  untwinned  it  resembles 
orthoclase,  but  is  optically  positive. 
Other  plagioclases  have  higher  re- 
fractive indices.  See  pages  31-32. 

Ottrelite  is  a  mineral  almost  exclusively 
confined  to  argillites  altered  by  dynamo- 
metamorphism.  It  occurs  in  leaves  and  plates 
and  usually  shows  hour-glass  structure.  Pleo- 
chroism may  be  rather  weak  or  wanting,  c=  yel- 
lowish green,  colorless,  b  =  blue,  a  =  olive-green. 
Cleavage  (001)  good.  The  low  double  refrac- 
tion and  high  relief  as  well  as  its  mode  of  occur- 
rence separate  it  from  all  other  minerals.  Zoi- 
site has  parallel  extinction  and  has  different 
color  and  smaller  optic  angle.  Clinozoi- 
site has  different  cleavage  (001:100  =  64°37'), 
and  usually  abnormal  interference  colors. 

Chalcedony  fills  or  lines  cavities  in  rocks,  or 
occurs  in  threadlike  aggregates,  concretionary 
masses,  or  spherulites.  It  is  insoluble  in  HC1. 
2V  =  10°^0°.  Z  e  o  1  i  t  e  s  do  not  have  thread- 
like habit  and  are  soluble  or  gelatinize  in  acid. 
Pseudochalcedony  is  negative  and  has 
a  small  value  for  2V. 


HIII  K-Fi'ltMIM.    MlNKRALS   AKD    ROCKS 


11 


The  Mineral  is  Negative 

'lontitc  li:is  an  extinction  angle  of  +  20°. 

:iinl  occurs  M-  -mall  |>risn»s  in  cavities  in 
basalts  and  other  basic  extrusive*,  in  pegma- 
tites. >ycliites.  etc.  From  e  p  i  s  t  i  1  li  i  t  e  it 

••arated  by  its  gelatinization  in  HC1  and  its 
extinction  angle.  ( '  a  n  c  r  i  n  i  t  e  has  higher 
birefringence  and  is  uniaxial.  t  h  o  m  s  o  n  i  t  e 
is  positive  and  has  higher  birefringence. 

Hi/l><-r*lhene  has  characteristic  pleochroism. 
c  =  greenish,  n  =  re<ldish  yellow,  b  =  pink,  fairly 
strong  in  iron-rich  specimens  but  In-coming 
fainter  in  liron/.ite.  The  extin<-tion  is  parallel, 
tint  sections  m  which  only  one  set  of  cleavage 
line*  is  brought  out  by  grinding  show  inclined 
extinction,  as  do  also,  of  course,  all  sections 
cutting  the  three  axes.  In  basal  sections  (these 
show  sharp  cleavage  lines  at  angles  of  approxi- 
mately 1M)°  with  each  other)  monoclinic 
I  >  y  r  o  x  e  n  e  s  show  the  emergence  of  an  axis 
while  orthorhombic  pyroxenes  show  the  emer- 
gence of  a  bisectrix.  The  cleavage,  (110): (110)  = 
'.M  Hi',  is  characteristic  of  all  pyroxenes.  Hyper- 
sthene  occurs  in  all  rocks  from  gabbroic  rocks  to 
granites.  Bronzite  is  positive  and  has 
weaker  pleochroism;  a  n  d  a  1  u  s  i  t  e,  with  similar 
pleochroism,  has  negative  elongation  and  different 
cleavage. 

Anorthite,  the  calcium  plagioclase,  shows 
polysynthetic  twinning  in  most  cases.  See 
pages  30-34. 

WoUastonile  has  (100)  good  and  (001)  dis- 
tinct cleavages  with  an  angle  between  them  of 
84.5°.  Extinction  angle  c: a  = +32°12'.  It  usu- 
ally occurs  in  tablets  or  rods  along  the  6  axis,  or 
reticulated  or  parallel  in  masses  in  granular 
limestones,  at  igneous  contacts  in  lime  rocks, 
but  very  rarely  in  igneous  rocks  themselves, 
then  usually  in  calcareous  inclusions.  It  gelat- 
ini/es  with  HC1.  Pectolite  and  tremo- 
1  i  t  e  differ  in  not  having  the  plane  of  the  optic 
axes  at  right  angles  to  the  elongation,  which  is 
very  cha  ract  erist  ic  of  sections  in  the  orthodiagonal 
zone  of  wollastonite.  P  i  s  t  a  c  i  t  e  has  higher 
refraction  and  higher  birefringence. 


The  Mineral  is  Positive 

Serpentine  is  always  secondary  and  occurs  as 
an  alteration  product  of  olivine,  less  commonly 
of  pyroxene  or  amphibole,  and  possibly  of 
other  ferromagnesian  minerals.  Antigorite 
is  the  mas>ive,  lamellar  variety;  here  is  included 
the  fibrous  variety.  Antigorite  is  nega- 
tive and  massive.  P  e  n  n  i  n  e  has  lower  double 
refraction,  usually  abnormal  interference  colors, 
and  is  pleochroic. 

Spodumene  has  typical  pyroxene  cleavage, 
has  extinction  c:c=  —23°  to  —  26°,  and  is  generally 
non-pleochroic  unless  the  sections  are  thick 
when  a  =  amethyst,  b  =  amethyst, c  =  colorless.  In 
many  cases  it  is  altered  to  a  mixture  of  albite  and 
muscovite.  It  occurs  in  pegmatite  veins,  often 
of  great  size,  and  in  granites  and  gneisses.  Pleo- 
chroism, moderate  birefringence,  and  occurrence 
separate  it  from  other  pyroxenes. 

Sillimanite  occurs  as  a  contact  mineral,  and 
in  long,  slender,  fine  needles  without  terminal 
faces  in  the  quartz  of  granites  and  gneisses.  May 
also  occur  in  prisms  or  aggregates  of  needles. 
Its  (010)  cleavage  is  perfect,  and  there  are 
transverse  fractures.  Apatite,  with  similar 
cross-parting,  has  much  lower  double  refraction 
and  negative  elongation.  Andalusite  is 
negative,  has  negative  elongation,  lower  bire- 
fringence, and  the  relation  of  the  axial  plane  to 
cleavage  is  different.  Scapolites  are  nega- 
tive, have  negative  elongation,  and  are  uniaxial. 
Z  o  i  s  i  t  e  has  weaker  double  refraction  and 
different  orientation. 

Anthophyllite,  an  orthorhombic  amphibole, 
usually  fibrous,  occurs  in  mica-  and  other  schists 
as  a  contact  mineral,  and  as  an  alteration  product 
of  olivine  in  serpentines,  gabbros,  peridotites, 
etc.  It  is  usually  non-pleochroic  in  thin  sec- 
tions, but  may  show  c  =  yellowish,  b  =  clove- 
brown,  reddish,  a  =  yellowish,  greenish,  colorless. 
Typical  amphibole  cleavage  and  parallel  extinc- 
tion separate  it  from  other  minerals.  In  basal 
sections  showing  sharp  cleavage  lines,  a  bisectrix 
appears  in  the  center  of  the  field ;  in  m  o  n  o  - 
clinic  amphiboles  this  lies  from  a  few 
degrees  to  twenty-two  to  the  side. 

Augile  is  usually  green,  brown,  reddish, 
violet,  or  yellowish,  but  is  rarely  colorless. 
Pyroxene  cleavage  and  high  extinction  angle 
(c:c=  —45°  to  —55°)  characterizes  it.  In  sections 
showing  parallel  extinction,  the  plane  of  the 
optic  axes  is  parallel  to  (010),  in  olivine  it  is 
parallel  to  (001).  Augite  is  a  common  pyroxene 
in  igneous  rocks.  It  also  occurs  in  metamorphosed 
sediments  and  metamorphosed  igneous  rocks. 


12 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The  Mineral  is  Negative 

Tremolite  has  typical  amphibole  cleavage, 
extinction  c :  c  =  — 16°,  and  occurs  as  crystals, 
long  or  short,  often  bladed  or  fibrous,  or  com- 
pact, in  metamorphosed  magnesian  limestones 
with  little  iron.  Where  iron  is  abundant,  actino- 
lite  occurs,  or  where  iron  is  the  only  carbonate, 
griinerite.  Tremolite  is  hardly  affected  by  HC1. 
Actinolite  is  pale  green  and  slightly  pleo- 
chroic,  wollastonite  gelatinizes  with  HC1 
and  has  the  trace  of  the  plane  of  the  optic  axes 
at  right  angles  to  the  elongation. 

Actinolite,  with  amphibole  cleavage,  extinc- 
tion c:c=  — 15°,  and  similar  in  habit  to  tremolite, 
is  rather  a  common  mineral  in  certain  schists  and 
metamorphosed  magnesian  limestones  containing 
much  ferrous  iron.  It  is  green  in  color  and  has  a 
faint  pleochroism,  sometimes  hardly  noticeable 
in  thin  sections,  green  to  yellowish  green. 

Epidote  (pistacite,  green  epidote)  is  a  com- 
mon contact  or  dynamo-metamorphic  mineral  in 
impure  calcareous  rocks,  and  a  secondary  mineral 
in  the  feldspars  of  many  igneous  rocks.  It  is 
often  associated  with  clinozoisite.  It  has  been 
described  as  primary  in  certain  granites.  It  is 
sometimes  very  abundant  with  quartz  in  the 
rock  called  epidosite.  Pistacite  is  the  iron-rich 
epidote,  clinozoisite  the  iron-poor  or  iron-free 
variety.  The  characteristic  pistachio  green  color, 
brilliant  interference  colors,  and  high  relief  sepa- 
rate it  from  all  other  minerals.  Pleochroism 
a  =  colorless  to  yellowish  or  greenish,  b  =  yellow- 
ish to  yellowish  gray,  c  =  green  to  light  yellowish 
brown,  sometimes  rather  weak.  The  plane  of 
the  optic  axes  lies  at  right  angles  to  the  elonga- 
tion of  the  crystal. 

Muscovite  has  a  characteristic  "bird's-eye 
maple"  appearance,  which  separates  it  from  all 
other  minerals  except  the  micas  and  talc.  The 
optic  axial  angle  (2E  =  60°-70°)  separates  it  from 
bleached  biotite  (2E  =  small  to  0°). 
Paragonite  can  be  separated  only  by 
chemical  tests.  Lepidolite  usually  has  a 
smaller  optic  angle  (2E  =  32°-84°),  but  in  some 
cases  may  not  be  distinguishable  except  by 
chemical  tests.  Talc  has  2E  =  6°-20°,  but  in 
shreds  it  cannot  be  distinguished  from  muscovite 
except  by  chemical  or  physical  tests,  or  by  asso- 
ciated minerals.  Primary  muscovite  never  occurs 
with  pyroxenes,  talc  usually  does.  The  fine, 
shredded  muscovite,  secondary  in  potash  feld- 
spars, is  called  s  e  r  i  c  i  t  e.  Do  not  call  the 
secondary  mica  in  plagioclase  sericite  unless  you 
are  certain  that  potash  is  present;  the  mica  prob- 
ably is  secondary  paragonite. 


The  Mineral  is  Positive 

Diallage  and  diopside  are  monoclinic  pyrox- 
enes. The  former  has  very  perfect  and  abun- 
dant (100)  cleavage  in  addition  to  the  (110) 
cleavage  of  the  latter.  Both  are  pale  green  to 
colorless,  and  have  extinction  angles  c:c=— 39°. 
Diopside  occurs  in  pyroxene-granites,  diorites, 
lamprophyres,  crystalline  schists,  and  magnesia- 
rich  marbles;  diallage  is  common  in  gabbros 
and  related  rocks,  and  in  peridotites  and  the  ser- 
pentines derived  from  them.  Pyroxene  cleavage 
separates  these  minerals  from  all  but  other  pyrox- 
enes, from  which  the  extinction  angle  separates 
them.  In  sections  showing  parallel  extinction, 
the  plane  of  the  optic  axes  is  parallel  to  (010),  in 
o  1  i  v  i  n  e  it  is  parallel  to  (001). 


Forsterite,  the  magnesia  olivine,  rarely  occurs 
in  igneous  rocks,  but  is  a  mineral  of  dynamic  and 
contact  metamorphosed  marbles,  basic  schists, 
and  gneisses.  It  is  colorless,  and  while  it  has  a 
distinct  cleavage  (010),  (001),  this  is  usually 
seen  only  as  heavy,  irregular  cracks.  The 
mode  of  occurrence  separates  forsterite  from 
olivine.  The  orientation  of  the  interference 
figure  (see  under  olivine)  separates  it  from  the 
pyroxenes. 

Olivine,  the  intermediate  magnesia-iron  va- 
riety, is  a  common  primary  mineral  in  basic 
rocks,  and  an  accessory  in  basic  schists,  gneisses, 
and  marbles.  It  alters  to  actinolite,  anthophyl- 
lite,  iddingsite,  magnetite,  chromite,  opal,  quartz, 
serpentine,  tremolite,  and  other  minerals.  Alter- 
ation to  serpentine  and  magnetite  are  most 
common.  It  gelatinizes  slowly  in  HC1.  The 
interference  figure  lies  parallel  to  (001),  while 
in  pyroxenes,  in  sections  showing  parallel 
extinction,  it  is  parallel  to  (010).  Fayalite 
has  2V  =  50°,  higher  birefringence,  and  is  nega- 
tive. Forsterite  has  a  different  mode  of 
occurrence. 


Pectolite  usually  occurs  in  tablets,  rods  along 
the  b  axis  which  are  rarely  terminated,  or  fibrous 
aggregates  of  acicular  crystals,  sometimes  radiat- 
ing. 2V  =  60°  and  c :  c  =  —  5°.  It  occurs  as  a 
secondary  mineral,  like  the  zeolites,  in  cavities  in 
basic  igneous  rocks,  sometimes  in  metamor- 
phosed rocks,  and  in  nephelite-syenites.  Wol- 
lastonite is  negative,  has  ( ± )  elongation, 
lower  birefringence,  and  higher  refractive  indices, 
and  c:a=+32°.  Pyroxenes  have  differ- 
ent orientation  of  the  interference  figure. 


OF    ROCK-FOKMIM;    Ml\KHU.s     \M)    RoCKS 


13 


The  Mineral  is  Negative 


iiti-,  the  white  soda  mica  analogous  to 
muscovite,  is  common  in  certain  schists  (paragonite- 
sehi>N>  aiul  probably  as  a  secondary  mineral  from 
plagiocla<e  (-ei-  under  muscovile).  It  cannot  be 
distinguished  from  muscovite  optically. 

])<itnlitf  rather  rarely  occurs  in  cavities  in 
diabases  and  basalts,  rarely  in  granites,  diorites; 
also  in  gneiss.  It  has  no  distinct  cleavage,  and 
c:a=l°~4°.  It  gelatini/es  with  HCl  and  gives 
reaction  for  boron.  Crcen  epidote  has 
different  color,  cleavage,  and  lower  birefringence. 

r)il<>ti»i>it<  is  paler  brown  than  biotite,  red- 
dish brown,  or  yellowish  brown,  sometimes 
greenish  or  colorless.  2V,  as  in  biotite,  is  small 
to  0°.  Biotite  has  stronger  pleochroism,  but 
when  bleached  may  not  be  distinguishable  from 
it.  I'hlogopite  is  essentially  a  mineral  of  marbles 
•ad'  crystalline  dolomites,  but  does  occur  in  the 
leucite  rocks  of  Wyoming  and  in  the  mica- 
peridot  itcs  of  southern  Illinois.  For  separation 
from  other  minerals,  see  under  muscovite. 

Lepidolite,  colorless  to  reddish,  pink,  or 
violet  .  in  many  cases  resembles  muscovite, 
from  which  its  optic  axial  angle  (2E  =  32°-84°), 
when  low,  may  separate  it.  The  optic  axial 
angle  also  separates  it  from  bleached 
biotite  (2V  =  small  to  0°).  In  most  cases  it 
can  \IG  separated  only  from  these  micas  by  the 
reaction  for  Li.  Lepidolite  occurs  in  granite- 
pegmatite  \eins,  greisen,  and  gneiss,  often  with 
tourmaline,  cassiterite,  etc. 

Fayalite,  the  iron  olivine,  may  be  colorless 
or  yellowish,  greenish,  reddish,  with  weak  or 
no  pleochroism  in  yellow  and  red  tones.  Oliv- 
ine has  2V  =  88°  (fayalite,  2V  =  ±50°),  is  opti- 
cally positive,  and  has  lower  birefringence. 
Forsterite  is  positive,  has  2V  =  86°,  lower 
indices,  and  different  mode  of  occurrence  (con- 
tact mineral  in  metamorphic  limestones). 

Talc,  orthorhombic,  closely  resembles  mus- 
covite in  thin  sections,  and  it  may  be  neces- 
sary to  use  chemical  means  to  separate  them. 
The  optic  angle  (2E  =  6°-20°)  is  smaller  than 
usual  in  mu-covite,  and  the  mode  of  occurrence  is 
different,  primary  muscovite  never  occurring 
with  pyroxene,  while  talc  commonly  does.  It 
has  the  same  "bird's-eye  maple"  appearance 
so  common  in  mica. 

Araganite,  under  the  microscope,  resembles 
calcite,  in  refractive  indices  and  double  refraction, 
but  it  is  biaxial  (2V  =  18°)  and  shows  no  cleavage. 
It  occurs  in  gypsum  deposits,  occasionally  in  ves- 
icules  in  basalt,  and  as  the  material  of  certain 
fossil  shells  and  corals.  For  chemical  separation 
from  calcite  see  Manual,  p.  568. 


The  Mineral  is  Positive 

Anhydrite  occurs  in  grains,  sharp  blades, 
seldom  in  threads  in  .sedimentary  beds  associated 
with  gypsum,  in  limestone,  or  with  rock  salt, 
and  rarely  in  cavities  in  lava  (Santorin).  Gyp- 
sum has  lower  double  refraction  and  —53° 
extinction,  d  i  s  t  h  e  n  e  has  lower  birefringence 
and  is  negative. 

Monazite  occurs  in  granites,  in  gneissoid 
rocks,  and  in  sediments,  but  most  commonly  in 
sands  and  gravels.  The  crystals  are  small, 
tabular  parallel  to  (100)  or  elongated  on  the  6 
axis,  rounded  grains,  occasionally  in  larger 
masses.  The  yellow,  non-pleochroic  color,  high 
birefringence,  and  high  relief,  separate  it  from 
most  minerals.  T  i  t  a  n  i  t  e  has  higher  bire- 
fringence, and  the  extinction  angle  is  39°  (mona- 
zite,  c:c  =  2°-6°).  Brookite  has  (±)  elonga- 
tion (monazite  negative),  and  2E  is  somewhat 
larger.  R  u  t  i  1  e  has  positive  elongation,  is 
usually  of  a  deeper  red  or  orange  color,  is  uni- 
axial,  has  higher  indices,  and  may  show  genicu- 
lated  or  heart-shaped  twins. 

Titanite  is  a  very  common  accessory  mineral 
in  the  acid  plutonites,  such  as  granites  and 
syenites,  abundant  in  nephelite-syenites,  and  less 
common  in  diorites.  It  is  also  abundant  in 
gneisses  and  schists,  and  in  some  limestones.  As 
a  secondary  mineral  (leucoxene)  it  is  derived  from 
titaniferous  magnetite,  ilmenite,  rutile,  and  other 
titanium-bearing  minerals.  The  pleochroism  is 
weak,  c>b>a,  in  brown  and  yellow  tones,  c:c» 
+39°,  and  2E  =  45°-68°.  The  strong  dispersion 
produces  colored  isogyres.  It  occurs  in  the  form 
of  prisms,  rhombs,  and  grains.  Monazite 
has  lower  birefringence,  smaller  extinction  angle, 
and  weak  dispersion.  Brookite  has  parallel 
extinction, 2V =0° to  —23°.  Rutile  is  uniaxial. 

Brookite  occurs  in  veins  with  various  other 
minerals — albite,  quartz,  nephelite,  garnets,  rutile, 
chalcopyrite,  galena,  etc. — and  in  gold  washings, 
always  in  the  form  of  crystals  of  various  habits, 
often  tabular.  The  acute  bisectrix  is  normal  to 
(100)  but  the  axial  plane  is  parallel  to  (001)  for 
red  and  yellow  and  parallel  to  (010)  for  green  and 
blue.  For  red  (670  w)  2E  =  58°0',  yellow  (589  w) 
2E  =  38°10',  yellowish  green  (555^)  2E  =  0°, 
green  (535-525  MM)  2E  =  21°40'-33°0'  (Manual, 
pp.  444-45,  and  Figs.  619-23).  Interference 
figure  for  white  light  is  a  combination  of  these, 
giving  a  peculiar  form  (Manual,  Fig.  (524). 
Cassiterite  and  rutile  have  different 
habit,  and  brookite  has  very  different  strength 
of  double  refraction  in  (100)  and  (010)  sections. 


14 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The    Mineral    i.    AN  I  SOT  R( 
The  Miner*!  it  NEGATIVE 

)PIC,      COLORED,  NON-I 
(-)•                                 'Th 
cre«*e  in  Index  of  Refracti 

LEOCHROIC.     UNIAX1AL. 
c  Mineral  U  POSITIVE  (+). 

o    01        oo               r*              r-                to              <o               w  S 

NM__                    -                   „                     -                  -                    »2 

In 
creas'g 
BIREFR. 

I 

Em.             o             m             o             in            oooe 
•  in              w             »             S             h;            <a      a,     o    * 

~i~                  -                "                -                U               —  '        ~      N-  N" 

^ 

Veu  High. 

High. 

Medium. 

Nol  Marked,  u 

<->  Nol  Marked. 

Medium. 

High. 

Van  High.. 

Eucol 

Apatite 

ite.  ^ 

-    Eudii 

T 

lite. 

v< 

suvianite 

.    ^ 

-T 

/\» 

015 

2j 

rcon. 

, 

090 
095 
.100 
120 

Cas 

siter 

ite.  . 

Magne 

Dolom 

160 
.200 
250 

Ruti 

e. 

_ 

HOCK-FORMING  MINERALS  AND  ROCKS 


15 


The  Mineral  is  Negative 

•'He  occurs  in  v;iri<ius  nephelite-syenites. 
It  h:is  very  weak  M)>K)  pleochroism  ur  none. 
Cleavage  (0001)  is  distinct.  Anomalous  'JI',  to 
50°.  Eudialyte  is  optically  positive,  and 
lias  negative  elongation.  Topaz  is  biaxial. 
Apatite  lias  poor  cleavage,  long  crystals 
-how  parting,  and  elongation  is  negative. 
M  e  1  i  1  i  t  e  has  characteristic  abnormal  Berlin 
blue  interference  color  and  basal  cleavage. 

Apatite  has  characteristic  basal  parting  in 
long  prisms.  It  is  a  very  common  accessory,  in 
the  form  of  small  prisms,  in  most  igneous  rocks. 
In  large  crystals  it  occurs  in  pegmatites,  some 
lamprophyres,  etc.  It  is  also  found  in  the  crys- 
talline schists,  limestones,  argillites,  etc.  Silli- 
m  unite  has  higher  double  refraction  and 
positive  elongation. 

Mil'lite  is  a  feldspathoid  and  does  not  occur 
in  quart /.-iH-aring  rocks.  It  usually  shows  ab- 
normal Berlin  blue  interference  colors.  The 
(001)  and  (110)  cleavages  are  poor;  only  the 
basal  cleavage  is  generally  seen  in  thin  sections, 
and  this  occurs  as  a  single  cleft  along  the  middle 
of  the  lath-shaped  section.  Peg  structure,  due 
to  inclusions  growing  inward  from  basal  sections, 
is  characteristic.  It  gelatinizes  with  HC1 
i  Manned,  p.  564).  Vesuvianite  and  z  o  i  - 
site,  both  of  which  may  give  the  abnormal 
interference  color,  are  insoluble  in  acids.  Vesuvi- 
anite has  higher  relief,  and  usually  occurs  as  a 
contact  mineral  in  limestone.  Zoisite  is  biaxial 
and  occurs  as  a  secondary  mineral. 

Vesuvianite  has  poor  (110),  (100)  cleavages. 
It  usually  occurs  as  a  contact  mineral  derived 
from  limestone,  but  has  also  been  found  in 
ancient  ejected  blocks  among  the  dolomite 
masses  of  Vesuvius.  In  some  cases  it  shows 
abnormal  blue  interference  colors,  or  biaxial 
character.  Zoisite  has  better  cleavage  and 
a  different  mode  of  occurrence. 

Calcite,  dolomite,  and  magnesite  cannot  be 
separated  under  the  microscope,  but  may  be  by 
chemical  means  (Manual,  p.  565).  A  r  a  g  o  n  - 
it  e  is  biaxial  with  2V  =18°,  and  it  differs  in 
certain  chemical  reactions  (Manual,  p.  568). 
B  r  u  c  i  t  e  differs  chemically  (Manual,  p.  567), 
and  has  much  lower  double  refraction.  Calcite 
is  .a  common  alteration  mineral  in  all  kinds  of 
igneous  rocks,  and  is  said  to  be  primary  in  some 
granites.  Both  calcite  and  dolomite  occur  as 
vein  minerals,  and  in  widespread  and  thick 
strata.  Magnesite  occurs  as  a  secondary  mineral 
derived  from  the  alteration  of  magnesia-bearing 
minerals.  It  also  occurs  in  talc-schists,  ser- 
pentines, etc.,  often  as  veins. 


The  Mineral  is  Positive 

l-'.nilinlyte  is  optically  positive  and  has  nega- 
tive elongation,  while  eucolite  is  negative 
and  has  po-itivr  elongation.  Anomalous  2E  may 
be  as  high  as  50°  (see  eucolite).  It  is  commonly 
associated  with  nephelite. 

///con  has  weak,  seldom  noticeable  pleo- 
chroism.  It  occurs  in  small  characteristic  crys- 
tals which  are  shorter  and  stouter  than  those  of 
apatite  and  which  have  brilliant  interference 
colors.  In  larger  grains  the  interference  colors 
are  very  high  and  pale,  and  the  mineral  is  brown- 
ish. Zircon  is  especially  common  in  acidic 
and  sodic  igneous  rocks,  but  is  also  found  in 
schists  and  gneisses,  and  as  a  residual  mineral 
in  decomposed  igneous  rocks. 

Cajusiterite  may  be  pleochroic  in  weak  brown- 
ish tones.  Cleavage  (110)  is  poor,  (100)  distinct. 
Gcniculated  twins  are  common.  It  occurs  as  a 
pncumatolytic  mineral  in  acid  dikes  and  quartz 
veins,  and  as  a  rare  primary  mineral  in  some 
igneous  rocks.  R  u  t  i  1  e  has  better  cleavage 
and  is  not  so  brown.  Anatase  is  negative. 
Brookite  is  biaxial.  Perofskite  is 
isotropic. 

Rutile  occurs  as  an  accessory  mineral  in 
granites,  syenites,  gneisses,  and  mica-schists, 
and  as  secondary  microlites  in  argillites.  It  is 
also  found  in  granular  limestones,  and  has  been 
found  forming  a  dike  with  apatite.  It  occurs  in 
grains,  sometimes  in  geniculated  twins,  though 
usually  in  acicular  crystals  in  quartz.  It  is 
also  found  regularly  intorgrown  in  phlogopite, 
biotite,  and  hematite,  in  so-called  s  a  g  e  n  i  t  e 
webs.  Pleochroism  seldom  noticeable  in  thin 
sections,  O  =  yellowish  to  brownish,  E  =  brownish 
yellow  to  greenish  yellow.  Cassiteritc 
has  lower  birefringence,  poorer  cleavage,  a  n  a  - 
t  a  s  e  is  negative  and  has  much  lower  bire- 
fringence, b  r  o  o  k  i  te  is  biaxial  and  has  different 
crystal  form,  and  perofskiteis  isotropic. 


16 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The   Mineral  it   ANISOTR 
The  Mineral  it  NEGATIVE 

OPIC,     COLORED,  NON-F 

(->.                                   The 
rease  in  Index  of  Refractio 

LEOCHROIC,     BIAXIAL 
Mineral  is  POSITIVE  (+). 

0000                     W                   O                   «                     0                   U)| 

in    o    ot     oo              t*             r-              to               to              in  « 
N"N    ~     -              "             -              "               -              ~3 

In 

oieas'j 
BIREFR. 

I 

EIA              o              in               o             in              c 
gin               (o               to                t-              r^               a 

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;o    o  o 
o»     o  « 

-"      N    N 

Verj   High. 

High. 

Medium. 

Not  Marked,  u 

"  Not  Marked. 

Medium. 

High, 

Very  High. 

Ka 

olin.  - 
ordieritc 

e.  ^        i 

—  Clii 

•  Zoisite 

ozoisite. 

Disthe 

1C.    — 

Antigc 

>rite.— 

—  • 

Clinochlc 

re. 

™  ••  Spo 

iumene. 

Hedenbt 

rgit 

• 

Gedrite 

^—  Sillimanite. 
_  Anthophyllitc. 

Actinol 

_ 

te.  

_ 

rtuj 

Diallag( 
Diopsid 

ite. 

e. 

Mu 
Pai 

scovite. 

agonite. 



Phlo 

gopite.  . 

onaz 

ite 

?ay 

alitc 

Tal< 

.090 
.095 
.100 
120 

nite 
] 

.160 
.200 
.250 

Tita 

iroo 

dte 

OF   ROCK-FORMING    MINERALS   AND   ROCKS 


17 


The  Mineral  is  Negative 

usually  appears  as  a  flour-like,  while, 
upaque  alteration  pro.lurt  of  feldspar.  It  may 
he  stained  yellow  or  red  l>y  iron  oxides.  When 
crvstalli/ed  it  occurs  in  the  form  of  leaves  or 
M-ales  with  an  extinction  angle  of  13°,  and  may 
l.e  mistaken  for  sericite,  although  its  l>ire- 
fringence  is  lower.  Much  so-called  kaolin  is 
colloidal  aluminium  silicate,  and  not  kaolin. 
M  u  s  e  o  v  i  t  e  (scricite)  and  h  y  d  r  a  r  g  i  1 1  i  t  e 
have  higher  birefringences. 

Cordierite,  orlhorhombic,  2V  from  40°-84°, 
•_'K  from  63°-150°,  occurs  in  gneisses  and  various 
schists,  rarely  as  a  primary  mineral  in  granites, 
andcsites,  etc.  As  a  metamorphic  mineral  it  is 
found  at  the  contact  of  acid  igneous  rocks  with 
-hales  and  slates.  When  treated  with  HF  it  gives 
characteriMic  prismatic  crystals  of  magnesium 
lluosilicatr  Pleochroic  halos  are  occasionally 
seen  in  sections  parallel  to  the  c  axis.  Trillings 
and  polysynthetic  twins  occur.  Quart/,  is 
uniaxial  and  positive,  a  1  b  i  t  e  is  jxwitive  and 
has  a  lower  index  of  refraction,  nephelite 
is  uniaxial  and  negative. 

Anlitjorite,  the  massive,  lamellar  serpentine, 
differs  from  common  serpentine,  which  is 
fil.rous,  in  being  negative,  and  in  its  habit. 
Pennine,  when  optically  negative,  is  sepa- 
rated by  its  optical  character,  when  positive,  by 
a  chemical  test  for  AljOj.  Pennine  also  has 
lower  birefringence,  usually  abnormal  inter- 
ference colors,  and  pleochroism.  Serpentines  are 
always  secondary  and  occur  as  an  alteration 
product  of  olivine,  less  commonly  of  pyroxene 
or  amphibole,  and  possibly  also  of  other  ferro- 
magnesian  minerals. 

Disthene  (cyanite)  does  not  occur  in  igneous 
rocks,  but  chiefly  in  muscovite  or  paragonite 
schists,  gneisses,  eclogites,  etc.,  often  associated 
with  garnets  or  corundum.  The  color  is  faint 
blue  hi  thin  sections,  in  some  cases  almost  color- 
less. Cleavages,  (100)  perfect,  (010)  distinct, 
making  an  angle  of  74°,  are  very  characteristic, 
although  they  do  not  show  in  all  sections.  Orien- 
tation, a  is  nearly  at  right  angles  to  (100),  c  is 
inclined  30°  on  (100)  to  the  edge  (100):  (010). 
S  i  1 1  i  m  a  n  i  t  e  and  andalusite  are  ortho- 
rhombic  and  have  different  cleavages,  topaz 
has  basal  cleavage  only,  z  o  i  s  i  t  e  usually  has 
abnormal  interference  colors  and  occurs  in  grains. 


The  Mineral  is  Positive 

Cliitozoinile  is  an  iron-poor  or  iron-free  epi- 
dote,  with  the  composition  of  zoisite.  It  is 
colorless  to  reddish  with  weak  or  no  pleochroism, 
has  extinction  angle  of  3°,  and  a  large  optic  angle 
(2V=80°-90°).  It  occurs  in  prisms  or  rods 
elongated  on  b,  and  in  grains.  Abnormal  inter- 
ference colors  are  common,  as  in  zoisite, 
but  zoisite  has  parallel  extinction  and  smaller 
optic  angle  (2V  =  0°H30°).  It  may  be  impos- 
sible to  separate  the  usual  grains  found  in  igneous 
rocks  from  /.oisit«.  Pistacite  lias  higher 
double  refraction. 

Bronzite  has  the  usual  pyroxene  cleavage, 
parallel  extinction  (see  under  hypersthene),  and 
is  slightly  pleochroic  in  green  and  pink  tones. 
2E=  ±  106°.  Hypersthene  has  similar  but 
stronger  pleochroism  and  is  negative.  Mono- 
clinic  pyroxenes  have  higher  bire- 
fringence and  inclined  extinction  in  sections  at 
right  angles  to  the  principal  optic  sections.  In 
basal  sections  which  show  sharp  cleavage  lines 
at  approximately  90°,  monoclinic  pyroxenes  show 
the  emergence  of  an  axis  while  orthorhomhic 
pyroxenes  show  the  emergence  of  a  bisectrix. 
E  n  s  t  a  t  i  t  e  is  non-pleochroic. 

Zoisite,  orthorhombic,  is  a  mineral  of  the 
crystalline  schist  formation,  produced  by  the 
dynarao-metamorphism  of  igneous  rocks  con- 
taining basic  plagioclase.  It  also  occurs  in  peg- 
matite dikes.  Abnormal  blue  interference  colors 
are  common.  Cleavage  (010)  good,  (100)  distinct. 
Clinozoisite  has  an  extinction  angle  of  3° 
and  an  optic  angle  of  2V  =  80°-90°,  while  zoisite 
has  an  angle  of  0°-60°.  M  e  1  i  1  i  t  e  gelatinizes 
with  acids,  occurs  only  in  quartz-free  rocks,  anil 
has  a  characteristic  habit.  Vesuvianite 
has  poorer  cleavage,  and  high  relief. 

Clinochlore,  one  of  the  chlorites,  occurs  in 
leaves,  scales,  plates,  or  leafy  aggregates  as  a 
mineral  of  schists  and  serpentines,  and  in  igneous 
rocks  from  the  alteration  of  ferromagnesian 
silicates.  2E  =  32°-90°,  c:C=-2°  to-9°,  maxi- 
mum birefringence  =0.011.  Pennine  is  (^), 
2E  =  0°— 61°,  has  parallel  extinction,  and  maxi- 
mum birefringence  of  0.002. 

Spodumene  has  typical  pyroxene  cleavage, 
extinction  c:c=  —  23°  to  —26°,  is  generally  non- 
pleochroic  unless  the  sections  are  thick,  when  a  = 
amethyst,  b  =  amethyst,  c  =  colorless.  In  many 
cases  it  is  altered  to  a  mixture  of  albite  and 
muscovite.  It  occurs  in  pegmatite  veins,  often 
of  great  size,  and  in  granites  and  gneisses.  Pleo- 
chroism, moderate  birefringence,  and  mode  of 
occurrence  separate  it  from  other  pyroxenes. 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The  Mineral  is  Negative 

Gedrite,  an  aluminium-bearing  orthorhombic 
amphibole,  occurs  in  metamorphic  schists  and 
gneisses,  and  as  a  contact  mineral.  It  is  usually 
pleochroic,  c  =  yellowish,  brownish,  b  =  clove- 
brown,  reddish,  a  =  yellowish,  greenish,  colorless. 
Anthophyllite,  the  other  orthorhombic 
amphibole,  has  2V  =  84°,  while  gedrite  has 
2V  =  57°-79°. 

Actinolite,  with  amphibole  cleavage,  extinc- 
tion c:c=  —15°,  and  similar  in  habit  to  tremolite, 
is  rather  a  common  mineral  in  certain  schists 
and  metamorphosed  magnesian  limestones  con- 
taining much  ferrous  iron.  It  is  green  in  color 
and  has  a  faint  green  to  yellowish  green  pleo- 
chroism,  sometimes  hardly  noticeable  in  thin 
sections. 

Muscovite  has  a  characteristic  "bird's-eye 
maple"  appearance,  which  separates  it  from  all 
other  minerals  except  the  micas  and  talc.  The 
optic  angle  (2E  =  60°-70°)  separates  it  from 
bleached  biotite  (2E  =  small  to  0°). 
Paragonite  can  be  separated  only  by 
chemical  tests.  Lepidolite  usually  has  a 
smaller  optic  angle  (2E  =  32°-84°),  but  in  some 
cases  may  not  be  distinguishable  except  by 
chemical  means.  T  a  1  c  has  2E  =  6°-20°,  but  in 
shreds  it  cannot  be  distinguished  from  muscovite 
except  by  chemical  or  physical  tests,  or  by  associ- 
ated minerals.  Primary  muscovite  never  occurs 
with  pyroxene,  talc  usually  does.  The  fine 
shredded  muscovite,  secondary  in  potash  feld- 
spars is  called  s  e  r  i  c  i  t  e  .  Do  not  call  the 
secondary  mica  in  plagioclase  sericite  unless  you 
are  certain  that  potash  is  present;  the  mica 
probably  is  secondary  paragonite. 

Paragonite  is  the  white  soda  mica  analogous 
to  muscovite.  It  is  common  in  certain  schists 
(paragonite-schists)  and  probably  as  a  secondary 
mineral  derived  from  plagioclase  (see  under  mus- 
covite). It  cannot  be  distinguished  from  musco- 
vite optically. 

Phlogopite  is  paler  brown  than  biotite, 
reddish  brown,  or  yellowish  brown,  sometimes 
greenish  or  colorless.  2V,  as  in  biotite,  is  small 
to  0°.  Biotite  has  stronger  pleochroism, 
but  when  bleached  may  not  be  distinguishable 
from  it.  Phlogopite  is  essentially  a  mineral  of 
marbles  and  crystalline  dolomites,  but  it  does 
occur  in  the  leucite  rocks  of  Wyoming  and  in  the 
mica-peridotites  of  southern  Illinois.  For  sepa- 
ration from  other  minerals,  see  under  muscovite. 


The  Mineral  is  Positive 

Hedenbergite  shows  typical  pyroxene  cleav- 
age. It  occurs  in  some  nephelite-  and  other  basic 
syenites.  2V  =  59°52',  c:c=-44°.  Separated 
from  other  pyroxenes  by  its  lower  double  refrac- 
tion and  by  its  extinction  angle.  O  1  i  v  i  n  e  has 
different  orientation  of  the  interference  figure. 

Sillimanite  occurs  as  a  contact  mineral,  and 
in  long,  slender,  fine  needles  without  terminal 
faces  in  the  quartz  of  granites  and  gneisses. 
It  may  also  occur  in  prisms  or  aggregates  of 
needles.  Its  (010)  cleavage  is  perfect,  and  there 
are  transverse  fractures.  Apatite,  with 
similar  cross-parting,  has  much  lower  double 
refraction  and  negative  elongation.  A  n  d  a  - 
1  u  s  i  t  e  is  negative,  has  negative  elongation, 
lower  birefringence,  and  the  relation  of  the  axial 
plane  to  the  cleavage  is  different.  S  c  a  p  o  - 
1  i  t  e  s  are  negative,  have  negative  elongation, 
and  are  uniaxial.  Z  o  i  s  i  t  e  has  weaker  double 
refraction  and  different  orientation. 

Anthophyllite,  an  orthorhombic  pyroxene,  usu- 
ally fibrous,  occurs  as  a  contact  mineral  in  mica- 
and  other  schists,  and  as  an  alteration  product 
of  olivine  in  serpentines,  gabbros,  peridotites,  etc. 
It  is  usually  non-pleochroic  in  thin  sections,  but 
may  show  c  =  yellowish,  b  =  clove-brown,  reddish, 
a  =  yellowish,  greenish,  colorless.  Typical  amphi- 
bole cleavage  and  parallel  extinction  separate 
it  from  other  minerals. 

Augite  is  usually  green,  brown,  reddish,  violet, 
or  yellowish,  but  rarely  colorless.  Pyroxene 
cleavage  and  high  extinction  angle  (c:c=— 45° 
to  —55°)  characterizes  it.  In  sections  showing 
parallel  extinction,  the  plane  of  the  optic  axes  is 
parallel  to  (010),  in  olivine  it  is  parallel  to 
(001).  Augite  is  common  in  igneous  and  meta- 
morphic rocks.  It  is  of  a  brownish-purplish  color 
when  titaniferous,  and,  of  that  color,  it  is  a  com- 
mon constituent  of  diabases  and  basalts. 

Diallage  and  diopside  are  monoclinic  pyrox- 
enes. The  former  has  very  perfect  and  abun- 
dant (100)  cleavage  in  addition  to  the  (110) 
cleavage  of  the  latter.  Both  are  pale  green  to 
colorless,  and  have  extinction  angles  of  c:c=  —39°. 
Diopside  occurs  in  pyroxene-granites,  diorites, 
lamprophyres,  crystalline  schists,  and  magnesia- 
rich  marbles;  diallage  is  common  in  gabbros 
and  related  rocks,  and  peridotites  and  the 
serpentines  derived  from  them.  Pyroxene  cleav- 
age separates  these  minerals  from  all  but  other 
pyroxenes,  from  which  the  extinction  angle 
separates  them.  In  sections  showing  parallel 
extinction,  the  plane  of  the  optic  axes  is  parallel 
to  (010),  in  olivine  it  is  parallel  to  (001). 


OF    ROCK-FORMINO    MINERALS   AND    ROOKS 


19 


The  Mineral  is  Negative 

Fayalitr.  the  iron  olivine,  may  !>«•  colorless  Of 
yellowish,  greenish,  reddish,  with  weak  or  no 
plcocliroism  in  yellow  and  red  tones.  O  1  i  v  i  n  e 
has  2V=SS°  (fayalite,  2V=*50°),  is  optically 
IMtsitive,  anil  has  lower  birefringence.  Fors- 
t  e  r  i  t  e  is  positive,  has  2V  =  80°,  lower  indices, 
an. I  different  mode  of  occurrence  (contact  in 
metamurphic  limestones). 

Talc,  orthorhombic,  closely  resembles  m  u  s  - 
covite  in  thin  sections,  and  it  may  l>e  neces- 
-ary  to  use  chemical  means  to  separate  them. 
The  optic  angle  (2E  =  6°-20°)  is  smaller  than 
usual  in  museovite,  and  the  mode  of  occurrence 
is  different,  primary  muscovite  never  occurring 
with  pyroxene,  while  talc  commonly  does.  It 
has  the  same  "bird's-eye  maple"  appearance 
so  common  in  mica. 


The  Mineral  is  Positive 

Olivine,  the  intermediate  magnesia-iron  va- 
riety, occurs  as  a  common  primary  mineral  in 
basic  rocks,  and  as  an  accessory  in  basic  schists, 
gneisses,  and  marbles.  It  alters  to  actinolite. 
anthophyllite,  iddingsite,  magnetite,  cliromite. 
opal,  quartz,  serpentine,  tremolite,  and  other 
minerals.  Alteration  to  serpentine  and  magnet- 
ite are  most  cojnmon.  It  gelatinizes  slowly  in 
HC1.  Th»  intnrf.  -miBiMnn  parallel  to 

(001)  while  in  pyroxenes,  in  sections  show- 
ing parallel  extinction,  it  is  parallel  to  (010). 
Fayalite  has  2V  =  50°,  higher  birefringence, 
and  is  negative.  Forsterite  has  a  different 
mode  of  occurrence. 

Monazite  occurs  in  granites,  in  gneissoid  rocks, 
in  sediments,  and  most  commonly  in  sands  and 
gravels.  The  yellow,  non-pleochrioc  color,  high 
birefringence  and  high  relief,  separate  it  from 
most  minerals.  T  i  t  a  n  i  t  e  has  higher  birefrin- 
gence, the  extinction  angle  is  39°  (monazite,  c:c  = 
2° -6°),  b rook ite  has  (±)  elongation  (monazite 
negative),  2E  is  somewhat  larger.  R  u  t  i  1  e 
has  positive  elongation,  is  usually  of  a  deeper 
red  or  orange  color,  is  uniaxial,  has  higher  indices, 
and  may  show  geniculated  or  heart-shaped  twins. 

Titanite,  in  the  form  of  prisms,  rhombs,  and 
grains,  is  a  very  common  mineral  in  acid  pluton- 
ites.  such  as  granites  and  syenites,  abundant  in 
nephelite-syenites,  and  less  common  in  diorites. 
It  is  also  abundant  in  gneisses  and  schists,  and 
in  some  limestones.  As  a  secondary  mineral 
(leucoxene)  it  is  derived  from  titaniferous  magne- 
tite, ilmenite,  rutile,  and  other  titanium-bearing 
minerals.  Pleochroism  is  weak  c>b>o,  in  brown 
and  yellow  tones,  c:c=+39°,  and  2E  =  45°-68°. 
Monazite  has  lower  birefringence,  smaller 
extinction  angle,  and  weak  dispersion.  Brook- 
i  t  e  has  parallel  extinction,  2V =0°  to  —23°,  ami 
rutile  is  uniaxial. 

Brookite  occurs  in  veins  with  various  other 
minerals  such  as  albite,  quartz,  nephelite,  garnets, 
rutile,  chalcopyrite,  galena,  etc.,  and  in  gold 
washings,  always  in  the  form  of  crystals.  The 
acute  bisectrix  is  normal  to  (100)  but  the  axial 
plane  is  parallel  to  (001)  for  red  and  yellow  and 
parallel  to  (010)  for  green  and-  blue.  For  red 
(670  MM)  2E  =  58°0',  yellow  (589  MM)  2E=38°10', 
yellowish  green  (555  MM)  2E  =  0°,  green  (535  to 
525  MM)  2E  =  21°40'-33°0'  (Manual,  pp.  444-^5, 
Figs.  619-23).  Interference  figure  for  white  light 
is  a  peculiar  combination  of  all  these  (Manual, 
Fig.  624).  Cassiterite  and  rutile  have 
different  habit,  and  brookite  has  very  different 
strength  of  double  refraction  in  (100)  and  (010) 
sections. 


20 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The  Mineral  U  ANISO1 
The  Mineral  is  NEGATIVE 

•ROPIC,    COLORED,    PLEO 
(-)•                                     The 
create  in  Index  of  Refractio 

CHROIC,    UNIAX1AL. 
Mineral  ..  POSITIVE  (+). 

a 

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In 
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BIREFR. 

4. 

E  w               o               in               o               in               oooo 
u;              v              i»              r~              cooiom 

•«—                  r-                  »-                  ^                  *-                  ^        _      r^      rj 

Verj  High. 

High. 

Medium. 

Not  Marked,  u 

ONot  Marked. 

Medium. 

High. 

Very  High. 

Apatite 

^ 

T 

3run 

Vesuvi 

um.  «^— 

anite.  ^ 

e.  ™ 

—  T 

010 
.015 

020 
.025 

T 

ourmalin 

e. 

.035 
.040 
.045 

Biotite. 

- 

/ 

nat; 

se. 

090 
095 
100 
.120 

_ 

Ht 

mat 

Magn 

Dolom 

.140 
160 
200 
250 

Ru 

tile. 

- 

te. 

1 

OF   ROCK-FORMI\<;    MtNKRALS   AND    ROCKS 


21 


The  Mineral  is  Negative 

Apatite  has  characteristic  basal  part  inn  in 
long  prisms.  It  i<  easily  soluble  in  II;S()4  and 
the  solution  gives  a  \ello\v  precipitate  with 
ammonium  molybdatc  (Mnininl,  p.  565).  Apa- 
tite, in  the  form  of  small  prisms,  is  a  very  ( unon 

accessory  in  most  igneous  rocks.  In  large 
cr\>tal-  it  nccur>  ill  pegmatites,  some  lampro- 
phyres.  etc.  It  is  also  found  in  crystalline 
schists,  limestones,  argillites,  etc.  S  i  1 1  i  m  a  n  - 
ite  has  higher  double  refraction  and  positive 
elongation. 

\t>!>titr  is  a  feldspathoid  and  does  not  occur 
in  quartz-bearing  rocks.  It  usually  shows  ab- 
normal blue  interference  colors.  The  (001) 
and  (110)  cleavages  are  poor;  only  the  Iwsal 
cleavage  is  generally  seen  in  thin  sections,  and 
this  occurs  as  a  single  cleft  along  the  middle  of 
the  lath-shai>ed  section.  Peg  structure,  due  to 
inclusions  growing  inward  from  basal  sections,  is 
characteristic.  It  gelatinizes  easily  with  HC1 

•ntal,  p.  564).  Ves  u  vi  an  i  t  e  and  zois- 
i  t  e  .  both  of  which  may  give  the  abnormal 
blue  interference  color,  are  insoluble  in  acids. 
Yesuvianite  has  higher  relief,  and  usually  occurs 
as  a  contact  mineral  in  limestone.  Zoisite  is 
biaxial  and  occurs  as  a  secondary  mineral. 

Vesuvianite  has  poor  (110),  (100)  cleavages. 
It  usually  occurs  as  a  contact  mineral  derived 
from  limestone,  but  has  also  been  found  in 
ancient  ejected  blocks  among  the  dolomite  masses 
of  Vesuvius  and  Monte  Somma.  In  some  cases 
it  shows  abnormal  Berlin  blue  interference  colors, 
or  biaxial  character.  It  is  insoluble  in  acids 
unless  first  fused.  Z  o  i  s  i  t  e  has  better  cleav- 
age and  different  mode  of  occurrence. 

Corundum  occurs  as  a  primary  mineral  in 
alumina-rich  igneous  rocks,  both  acid  and  basic, 
such  as  pegmatites,  syenites,  anorthosites,  and 
dunites.  It  is  rare  as  a  contact  mineral,  but 
occurs  in  granular  limestones  and  dolomites, 
gneisses,  mica-schists,  etc.  The  pleochroism, 
O  =  blue,  red,  E= sea-green,  yellow,  or  greenish 
yellow,  is  seen  only  in  deeply  colored  specimens. 
It  has  a  poor  parting  (1011),  (0001).  The  high 
relief  separates  it  from  similar  minerals  except 
vesuvianite  from  which  it  is  separated  by 
its  hardness,  higher  double  refraction,  and  by 
chemical  means. 


The  Mineral  is  Positive 

Kutile  occurs  as  an  accessory  mineral  in 
granites,  syenites,  gneisses,  and  mica-schists,  and 
as  secondary  microlites  in  argillites.  It  is  also 
found  in  granular  limestones,  and  has  been 
found  forming  a  dike  with  apatite.  It  occurs  in 
grains,  sometimes  in  gcniculatcd  twins,  though 
usually  in  acicular  crystal-  in  quart/.  It  is  also 
found  regularly  intergrown  in  phlogopite,  bio- 
t  it  e,  and  hematite,  in  so-called  sagenite- 
webs.  Pleochroism  seldom  noticeable  in  thin 
sections,  O«  yellowish  to  brownish,  E»  brownish 
yellow  to  greenish  yellow.  Cassiterite 
has  lower  birefringence,  poorer  cleavage;  ana- 
t  a  s  e  is  negative  and  has  much  lower  birefrin- 
gence; brookite  is  biaxial  and  has  different 
crystal  form,  and  pcrofskite  is  isotropic. 


22  ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 

The  Mineral  is  Negative 

Tourmaline,  a  pneumatolytic  mineral,  occurs 
in  granites  and  pegmatites,  and  rocks  in  contact 
with  these,  in  schists,  gneisses,  talc-schists,  and 
limestones  and  marbles.  That  found  in  marble 
is  usually  brown;  in  greisen  and  with  tin  ores 
usually  blue-black;  and  in  association  with 
lepidolite  red,  yet  these  colors  are  not  confined 
to  the  rocks  mentioned,  and  two  colors  may 
occur  together.  Red  and  green  transparent, 
and  black  opaque  varieties  also  occur.  It  forms 
prisms,  grains,  and  needles,  the  latter  in  many 
cases  in  radiating  groups,  so-called  tourmaline 
suns,  but  the  needles  are  not  necessarily  con- 
fined to  one  plane,  but  may  radiate  in  all  direc- 
tions, giving  in  thin  sections  a  central  portion 
showing  basal  sections,  characteristically  three-, 
six-,  nine-sided,  etc.,  often  zonal,  surrounded  by 
radiating  crystals.  The  uniaxial  character  and 
the  strong  pleochroism,  greatest  in  the  direction 
at  right  angles  to  the  vibration  direction  of  the 
lower  nicol,  separate  it  from  all  other  minerals, 
most  of  which  are  dark  when  the  elongation  is 
in  the  direction  of  vibration  of  the  lower  nicol. 

Biotite  is  a  common  mineral  of  the  acid  and 
intermediate  rocks,  both  plutonic  and  extrusive, 
and  of  some  of  the  lamprophyres,  and  occurs  as  a 
metamorphic  mineral  in  gneisses,  schists,  and 
various  other  rocks.  In  many  cases  it  is  inter- 
grown  with  muscovite,  either  in  parallel  inter- 
growth  or  with  muscovite  forming  the  outer  zone. 
Pleochroism,  strong  c<b>a;  c  and  b  =  deep 
brown  to  red-brown,  deep  green,  a  =  light  yellow 
to  reddish,  light  greenish.  It  has  a  golden 
brown  color  in  some  nephelite-syenites.  Basal 
sections  are  non-pleochroic  and  give  nearly  to 
quite  uniaxial  figures.  "Bird's-eye  maple" 
appearance  is  characteristic  of  all  micas.  Pleo- 
chroic  halos  about  minute  inclusions  of  zircon, 
etc.,  are  common  (Manual,  p.  323).  Tour- 
maline is  darkest  when  its  long  direction  is 
at  right  angles  to  the  polarizer,  lepidolite 
is  non-pleochroic,  zinnwaldite  has  less 
pleochroism  and  occurs  in  greisen  and  with  tin 
deposits,  but  may  require  test  for  Li  to  distinguish, 
phlogopite  is  less  pleochroic  and  generally 
occurs  in  crystalline  limestones.  Horn- 
blende is  biaxial,  has  inclined  extinction,  and 
does  not  show  the  "bird's-eye  maple"  effect. 


or  l;>"  k-l-'.'UMiSi.  MIM.UM.S  AMI  ROCKS  23 

The  Mineral  is  Negative 

.ln/1/n.vr  occurs  in  pyramids  .-mil  taMets,  and 
is  found  in  some  granite-pegmatites.  I'  usually 
lias  pleochroism,  O  =  deep  liluc  or  orange-yellow. 
K  =  light  blue  or  light  yellow,  lull  it  may  l>i-  very 
weak,  M>  thai  tin-  mineral  appears  colorless  in 
(liin  sections.  Colorless  or  yellow  portions  arc 
usually  normal,  while  Niie  portions  show  anoma- 
lous opening  of  the  interference  cross  and  do 
not  fully  extinguish.  I*  e  r  o  f  s  k  i  t  c  differs 
in  form  and  the  anomalous  interference  colors 
are  lower. 

Ctilciti.  ilnliiniiti-,  and  magnesite  cannot  be 
separated  under  the  microscope,  hut  may  lx-  liy 
chemical  means  (Manual,  p.  565).  A  r  a  %  o  n  - 
ite  is  liiaxial  with  2V=  18°,  and  differs  in 
certain  chemical  reactions  (Manual,  p.  568). 
Hrucite  differs  chemically  (Manual,  p.  567), 
and  has  much  lower  doultle  refraction.  Calcite 
is  a  common  alteration  mineral  in  all  kinds  of 
rocks,  ami  is  said  to  l>e  primary  in  some  granites. 
Roth  ealcite  and  dolomite  occur  as  vein  minerals, 
and  in  widespread  and  thick  strata.  Magnesite 
occurs  as  a  secondary  mineral  from  magnesia- 
liearing  varieties.  It  also  occurs  in  talc-schists, 
serpentines,  etc.,  often  as  veins. 

Siilerite  has  higher  indices  of  refraction 
than  the  preceding  three  carbonates,  and  is 
usually  somewhat  yellowish  or  brownish.  It  is 
a  common  mineral  of  ore  veins  and  of  limestones. 
It  is  also  found  in  gneisses,  slates,  shales,  gray- 
wackes,  etc. 

Hi' mat  ite  is  found  in  rocks  of  all  kinds,  either 
as  small  hexagonal  crystals,  rare  in  igneous  rocks, 
as  pseudomorphs  after  magnetite,  a*  rims  around 
magnetite,  as  an  alteration  product  from  various 
ferromaitnesian  minerals,  and  as  stains  in  cleav- 
age cracks.  Also  in  immense  deposits  among 
sedimentarics.  Pleochroism,  ()  =  brownish  red, 
E  =  light  yellowish  red,  not  seen,  of  course,  in 
basal  sections  nor  in  earthy  varieties.  Mag- 
netite is  black  by  incident  light,  hematite 
red;  1  i  m  o  n  i  t  e  is  usually  yellow,  though  it 
may  be  red,  in  which  case  it  may  be  confused 
with  hematite.  In  such  cases  it  is  customary  to 
speak  of  the  material  as  red-,  brown-,  or  yellow 
iron  oxide.  Basal  sections  of  some  biotite  may 
appear  blood-red  and  closely  resemble  hematite, 
but  the  interference  figure  of  hematite  shows 
more  rings  in  sections  of  the  same  thickness. 


24 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The    Mineral    it    ANISOTR 
The  Mineral  ii  NEGATIVE 

OPIC,     COLORED,    PLEO 
(-)•                                   Th« 
rea»e  in  Index  of  Refractk 

CHROIC,      BIAXIAL 
Miner*!  it  POSITIVE  (+). 

<                                                           ' 

So    o       o               in              o              in              o              m  c 
o    ff>       oo               r*              f*              <e              <c              m  « 

M      N      —  ^     ~                     ~                   •«                    -                    -                    -  -J 

In 

citrig 

BIREFR, 

1 

Pin                o               m               o              in              oooo 
«ji/>                tf               to               r*               r*              cooiOin 

-;   —                 —                ^                «                p^                *.'^MN 

V«rj   High 

High. 

Medium. 

Not  Marked.u 

0  Not  Marked. 

Medium. 

High. 

Very  High. 

-  ± 

- 

Pennine 

„  « 

iiebeckit 
-  Thulit 

( 

orditrite 

nligorite 

usite. 

Quartz. 

c. 

mortieri 
n,- 

sthene.  • 

relite." 

Slauroli 

Distht 
Hyper 

Li 

"^Anda 

Clinochl 

>re. 

Comm 

on  hornblende    • 
Glaucopham 

— 

—  Spo 

dumenc. 

Barkev 

ikite.  1" 

Bas 

Ge 

altic  hor 

drite. 
nblende. 

^   Anlhc 

phyllit*. 

Actinoli 

te. 

Dullaf 

lea 

„  Aegiri 

e-augite 

Pistacite 



035 
040 

Titanoli 

'Orthite 
vine. 

Pai 
Zin 

agonitt 
nwaldite 

L 

f>hl< 

Biotite 
>gopite.  . 

.045 
.050 
.055 
.060 

Fay 

alile 

1 

\egirite. 

Semite 

trophylJ 

P 

edrr 

onti 

(e.  « 

.075 
.080 

.085 
.090 
.095 
.100 
.120 
.140 
.160 
.200 
.250 

nite 

Titi 

Bro 

okit 

7 

(IK    HIM  K-KoKMINC    MINERALS   AND    ROCKS 


25 


The  Mineral  is  Negative 

Cordiiritr,  orthorhombie,  2V  fnmi  40°  to  84°, 
21".  from  tiH0  to  l.'iii  .  occurs  in  gneisses  and  various 
schists,  rarely  as  a  primary  mineral  in  granites, 
andesiies.  i-tc.  As  a  metamurphic  mineral  it  is 

found  at  the  ( tact  of  acid  igneous  rucks  witli 

shales  and  slates.  I'leochroie  halos  are  occasion- 
ally seen  in  sections  parallel  to  the  c  axis.  Tril- 
lings and  polysynthetic  twins  occur.  Quart  / 
is  uniaxial  and  positive,  alhite  is  positive  and 
ha*  lower  indices  of  refraction,  and  n  e  p  h  e  1  i  t  e 
is  uniaxial  and  negative. 

l)ist>  mite)  does  not  occur  in  igneous 

nicks.  l>ut  chiefly  in  muscovite-  and  paragon  it  <> 
schist-,  gneix-cs,  cclogites.  etc.,  often  associated 
with  garnets  or  corundum.  The  color  is  faint 
blue  in  thin  sections,  in  some  cases  almost  color- 
less. Cleavages,  (100)  perfect,  (010)  distinct, 
making  an  angle  of  74°,  are  very  characteristic, 
although  they  do  not  show  in  all  sections.  Orien- 
tation, a  is  nearly  at.  right  angles  to  (100),  c  is 
inclined  30°  on  (100)  to  the  edge  (100):  (010). 
S  i  1  1  i  in  a  n  i  t  e  and  andalusite  are  ortho- 
rhombic  and  have  different  cleavages,  topaz 
has  basal  cleavage  only,  zoisite  usually  has 
abnormal  interference  colors  and  occurs  in 
grains. 

Diininrtierite  occurs  in  a  few  gneisses  and 
similar  rocks.  It  is  characterized  by  its  pleo- 
chroism,  a  =  blue,  b  =  yellowish,  reddish  violet, 
greenish,  c  =  colorless;  parallel  extinction;  good 
i  1 1» i)  cleavage;  and  small  optic  angle,  2V  =  30°. 
Blue  amphibole  is  monoclinic,  anda- 
lusite and  hypersthene  have  pink  to 
green  pleochroism,  staurolite  has  higher 
relief,  is  positive,  and  is  pleochroic  in  brown 
tones.  Spodumene,  when  pleochroic,  has 
amethystine  colors  and  is  positive. 

A/iilnhi.-iili  has  characteristic  though  fre- 
quently faint  pleochroism,  a  =  rose,  b  =  c  =  color- 
less to  light  green,  resembling  that  seen  in 
hypersthene.  Hypersthene,  however,  has 
positive  elongation  and  more  marked  cleavage. 
Andalusite  frequently  appears  in  irregular  grains, 
or  in  more  or  less  irregular  oval  forms  associated 
with  grains  of  magnetite  in  schists.  In  the 
variety  chiastolite  the  inclusions  are  found 
in  regular  arrangement  in  the  forms  of  rhombs, 
crosses,  etc.  in  cross-sections  and  parallel  to 
the  long  axes  of  prisms,  and  the  material  is 
altered  to  a  mica-like  mineral.  The  higher 
relief  separates  andalusite  from  cordierite. 
Andalusite  is  found  in  a  few  granites,  but  is 
essentially  a  mineral  of  slates,  schists,  and 
gneisses.  As  chiastolite  it  is  a  contact  mineral 
in  argillites  near  granitic  contacts. 


The  Mineral  is  Positive 

Pennine,  one  of  the  chlorites,  occurs  as  an 
alteration  product  of  biotite  ami  other  ferro- 
magnesian  minerals.  It  is  usually  nearly  uni- 
axial, usually  negative,  sometimes  positive,  green 
in  color  with  distinct  pleochroism,  b  and  a -green. 
c  =  yellowish,  and  with  parallel  extinction.  Ab- 
normal Herlin  blue  interference  colora^re  com- 
mon. Habit:  leaves,  scales,  leafy  aggregates, 
etc.  Micas  have  higher  birefringence  and 
different  pleochroism,  serpentine  has  higher 
birefringence  and  lower  indices  and  is  seldom 
pleochroic.  The  separation  from  a  n  t  i  g  o  r  i  t  e 
may  be  difficult  and  in  some  cases  may  l>e  ]x>ssible 
only  chemically. 

Riebeckite,  an  iron-  and  alkali-rich  amphibole, 
occurs  in  igneous  rocks  rich  in  soda  and  iron, 
such  as  alkali-granites,  and  in  metamorphosed 
igneous  rocks  and  sediments.  Extinction,  r:a  = 
5°.  Pleochroism,  a  =  deep  blue,  b  =  lighter  blue, 
c  =  yellowish  green.  Riebeckite  is  separated  from 
other  minerals  by  its  amphibole  cleavage,  from 
other  amphiboles  except  glaucophane,  gastaldite, 
and  arfvedsonite,  by  its  blue  color.  Pleochroism 
in  glaucophane  is  in-  violet  tones,  in 
arfvedsonite  greenish  blue  and  lavender. 
The  latter  also  has  a  higher  extinction  angle,  and 
both  have  higher  birefringence  than  riebeckite, 
though  this  may  be  concealed  by  the  deep  color 
yet  indicated  by  the  numl>er  of  rings  in  the 
interference  figure  (see  top  of  page  39). 

Thulile,  the  manganese  zoisite,  has  a  faint 
pleochroism,  a  =  nearly  colorless,  b  =  rose,  c= yel- 
lowish. It  occurs  in  pegmatites  and  in  crystal- 
line schists.  Zoisite  is  non-pleochroic, 
andalusite  is  optically  negative,  and 
green  epidote  has  much  higher  bire- 
fringence. 

Bronzite  has  the  usual  pyroxene  cleavage, 
parallel  extinction  (see  bottom,  page  37),  2E  = 
=*=106°,  and  it  is  slightly  pleochroic  in  pink  and 
green  tones.  Hypersthene  has  similar  but 
stronger  pleochroism  and  is  negative.  Mono- 
clinic  pyroxenes  have  higher  birefrin- 
gences and  inclined  extinction  in  sections  at  right 
angles  to  the  principal  optic  sections.  In  basal 
sections  (which  have  sharp  cleavage  lines  at 
angles  of  approximately  90°  with  each  other) 
monoclinic  pyroxenes  show  the  emergence  of  an 
axis  while  orthorhombic  pyroxenes  show  the 
emergence  of  a  bisectrix.  Enstatite  is  non- 
pleochroic. 


26 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The  Mineral  is  Negative 

Antigorite,  the  massive,  lamellar  serpentine, 
differs  from  common  serpentine,  which 
is  fibrous,  in  being  negative,  and  in  its  habit. 
Pennine,  when  optically  negative,  is  sepa- 
rated by  its  optical  character,  when  positive,  by 
a  chemical  test  for  AUOs.  Pennine  also  has 
lower  biijgfringence,  usually  abnormal  interference 
colors,  and  pleochroism.  Serpentine  is  always 
secondary  and  occurs  as  an  alteration  product 
of  olivine,  less  commonly  of  pyroxene  or  amphi- 
bole,  and  possibly  also  of  other  ferromagnesian 
minerals. 

Hypersthene  has  characteristic  pleochroism, 
c  =  greenish,  a  =  reddish  yellow,  b  =  pink,  fairly 
strong  in  iron-rich  specimens  but  fainter  in 
bronzite.  The  extinction  is  parallel,  but  sections 
in  which  only  one  set  of  cleavage  lines  is  brought 
out  by  grinding  show  inclined  extinction,  as  do 
also,  of  course,  all  sections  cutting  the  three  axes. 
In  basal  sections  (these  show  sharp  cleavage  lines 
at  approximately  right  angles  to  each  other) 
monoclinic  pyroxenes  show  the  emer- 
gence of  an  axis  while  orthorhombic  pyroxenes 
show  the  emergence  of  a  bisectrix.  The  cleav- 
age, (110):(1TO)=91°40',  is  characteristic  of  all 
pyroxenes.  Hypersthene  occurs  in  all  rocks 
from  the  gabbro  family  to  granites.  Bronz- 
ite is  positive  and  has  weaker  pleochroism; 
andalusite,  with  similar  pleochroism,  has 
negative  elongation  and  different  cleavage. 

Common  hornblende,  a  widespread  monoclinic 
mineral  in  acid  and  intermediate  igneous  rocks, 
is  strongly  pleochroic  in  green,  rarely  brown,  tones. 
Pargasite  is  the  name  applied  to  the  green 
varieties,  common  hornblende  to  the  brown  and 
black  varieties,  although  the  name  hornblende 
is  applied  to  all.  Hornblende  occurs  also  in 
limestones  and  is  widespread  among  the  schists 
and  other  metamorphic  rocks.  Arfvedsonite 
has  negative  elongation ;  other  amphiboles 
have  different  extinction  angles.  Amphibole 
cleavage  separates  hornblende  from  all  other 
minerals. 

Glaucophane,  with  (110):(1TO)=55°16',  re- 
sembles amphibole  in  habit.  It  occurs  in  grains, 
prisms,  and  fibers.  The  pleochroism  is  character- 
istic, a  =  nearly  colorless  to  yellowish  green, 
b  =  reddish  to  bluish  violet,  c  =  blue,  c :  c  =  —  4°  to 
-6°,  2V  =  45°,  2E  =  85.5°,  but  the  mineral  some- 
times appears  nearly  uniaxial.  Glaucophane  is  a 
metamorphic  mineral  of  mica-schists,  amphibo- 
lites,  and  gneisses,  especially  those  derived  from 
basic  rocks  which  formerly  contained  much  soda. 
Garnets,  mica,  omphacite,  epidote,  zoisite,  etc., 
are  frequent  associates. 


The  Mineral  is  Positive 

Ottrelite  is  a  mineral  almost  exclusively 
confined  to  argillites  altered  by  dynamo- 
metamorphism.  It  occurs  in  leaves  and  plates, 
and  usually  shows  hour-glass  structure.  Pleo- 
chroism may  be  rather  weak  or  wanting,  c  = 
yellowish  green,  colorless,  6  =  blue,  a  =  olive- 
green.  Cleavage  (001)  good.  The  low  double 
refraction  and  high  relief,  as  well  as  the  mode  of 
occurrence,  separate  it  from  all  other  minerals. 
Zoisite  has  parallel  extinction,  different  color, 
and  smaller  optic  angle.  Clinozoisite  has 
different  cleavage  (001:100  =  64°37'),  and  usually 
abnormal  interference  colors. 

Staurolite  occurs  in  crystalline  schists  as  a 
contact  or  dynamo-metamorphic  mineral.  In- 
clusions symmetrically  arranged  or  in  subparallel 
bands  are  common.  The  yellow,  red,  or  brown 
color,  the  pleochroism,  c  =  red-brown,  a  and  b  = 
yellow,  the  parallel  extinction,  the  large  optic 
angle,  2V  =  89°,  giving  a  straight  bar  in  sections 
at  right  angles  to  an  optic  axis,  separate  it  from 
other  minerals.  Andalusite  is  negative  and 
has  different  pleochroism,  g  e  d  r  i  t  c  has  smaller 
optic  axial  angle  (2V  =  57°-79°)  and  amphibole 
cleavage,  anthophyllite  has  amphibole 
cleavage  and  is  rarely  pleochroic  in  thin  sections, 
vesuvianite  has  a  pale  yellow  color,  is 
negative,  and  has  lower  birefringence. 

Clinochlore,  one  of  the  chlorites,  occurs  in 
leaves,  scales,  plates,  or  leafy  aggregates  as  a  min- 
eral of  schists  and  serpentines,  and  as  a  secondary 
mineral  in  igneous  rocks  from  the  alteration  of 
ferromagnesian  silicates.  2E  =  32°-90°,  c :  c  =  —  2° 
to  — 9°,  maximum  birefringence  =  0.011.  Pen- 
nine is  (=*=),  2E  =  0°-6T,  has  parallel  extinction, 
and  the  maximum  birefringence  is  0.002. 

Spodumene  has  typical  pyroxene  cleavage, 
extinction  c:c=  —  23°  to  —26°,  is  generally  non- 
pleochroic  unless  the  sections  are  thick,  when 
a  =  amethyst,  b  =  amethyst,  c  =  colorless.  In 
many  cases  it  is  altered  to  a  mixture  of  albite  and 
muscovite.  It  occurs  in  pegmatite  veins,  often 
in  very  large  crystals,  and  in  granites  and  gneisses. 
Pleochroism,  moderate  birefringence,  and  mode 

of  occurrence  separate  it  from  other  pyroxenes. , 

*~»^>^j-- 
Anthophyllite,    an    orthorhombic    pyroxene, 

usually  fibrous,  occurs  in  mica-  and  other  schists 
as  a  contact  mineral,  and  as  an  alteration  product 
of  olivine  in  serpentines,  gabbros,  peridotites,  etc. 
It  is  usually  non-pleochroic  in  thin  sections,  but 
may  show  c  =  yellowish,  b  =  clove-brown,  reddish, 
a  =  yellowish,  greenish,  colorless.  Typical  amphi- 
bole cleavage  and  parallel  extinction  separate  it 
from  other  minerals. 


I;...  K-|  ..I:MI\(.   Mi\mu.s   \\n  ROCKS 


27 


The  Mineral  is  Negative 

Arfirdmtniti;  rather  a  ran'  mineral,  occurs  iti 
soda-bearing  igneous  rocks.  MX  nephelite-syemtes, 
phonolites.  tingiiaites.  pantellerites,  and  alkali- 
pegmatites.  It  lias  ainpliiliolc  cleavage  and  is 
characterize!  by  .strong  plt'ocliroisni,  a-  pair 
greenish  yellow.  b  =  lavender,  o  ^  deep  greenish 
blue.  Absorption  C>b>0.  2V  is  large,  and  the 
mineral  is  probably  optically  |x>sitive.  c:c  on 

nii)]  =  i4°.  Glauoophane  has  positive 
elongation  ami  c:f  =  —  I'  to  —  0°.  II ic beck  i  t  c 
has  c:c=  — 85°  (c:a  =  5°),  aegiritc  has  differ- 
ent color,  b  a  r  k  e  v  i  k  i  t  <•  has  2V  =  about  54°, 
c:c=  11°.  and  the  color  is  brown. 

H<irkirikiti-.  a  nire  brown  ainphilx>lc,  has 
c:c=14°,  and  is  strongly  pleochroic  in  brown 
tones.  A  sharp  separation  between  barkcvikite 
and  basaltic  hornblende  is  not  possible.  Other 
amphit>oles  differ  as  mentioned  under  arfved- 
sonite. 

ti\  an  aluminium-bearing  orthorhombic 
amphibolc.  occurs  in  metamorphic  schists  and 
gneisses,  and  as  a  contact  mineral.  It  is  usually 
pleochroic,  c  =  yellowish,  brownish,  b  =  clove- 
brown.  reddish,  a  =  yellowish,  greenish,  colorless. 
A  n  t  h  o  p  h  y  1 1  i  t  e  ,  the  other  orthorhombic 
amphibolc.  has  2V  =  84°,  while  gedrite  has  2V  = 
57°-79°. 

Basaltic  hornblende  is  common  in  basic 
extrusives.  It  frequently  shows  absorption  rims. 
It  is  pleochroic  in  strong  brown  and  yellow  tones, 
with  c>b>a,  also  in  green  and  brown  tones  with 
a  =  green,  b  and  c  =  brown.  Common  horn- 
blende has  c:c  =  —12°  to  —20°,  sometimes 
positive  character,  lower  double  refraction,  and 
sometimes  lower  2V.  B  i  o  t  i  t  e  generally  has 
the  ''bird's-eye  maple"  effect.  See  under  arfved- 
sonite. 

Actinolite,  with  amphibole  cleavage,  extinc- 
tion c:c=  —15°,  and  a  habit  similar  to  tremolitc, 
is  rather  a  common  mineral  in  certain  schists  and 
metamorphic  magnesian  limestones  containing 
much  ferrous  iron.  It  is  green  in  color  and  has 
a  faint  green  to  yellowish  green  pleochroism. 
sometimes  hardly  noticeable  in  thin  sections. 


The  Mineral  is  Positive 

Augite  is  usually  green,  brown,  reddish,  violet, 
or  yellowish,  but  rarely  colorless.  Pyroxene 
cleavage  and  high  extinction  angle  (c:c«-  —45°  to 
—  .W)  characterize  it.  In  sect  inns  showing  paral- 
lel extinction,  the  plane  of  the  optic  axes  j<  parallel 
to  (010),  in  olivine  it  is  parallel  to  (001). 
Augite  is  a  common  pyroxene  in  igneous  rocks,  and 
it  also  occurs  in  metamorphosed  sediments  and 
igneous  rocks.  It  is  of  a  brownish-purplish 
color  when  titaniferous,  and.  with  that  color,  it  is 
a  common  constituent  of  diabases  and  basalts. 

Diallage  and  diopxide  are  monoclinic  pyrox- 
enes. The  former  has  very  perfect  and  abundant 
(100)  cleavage  in  addition  to  the  (110)  cleavage 
of  the  latter.  Both  are  pale  green  to  colorless, 
and  have  extinction  angles  of  c:c=  —39°.  Diop- 
side  occurs  in  pyroxene-granites,  diorites,  lampro- 
phyres,  crystalline  schists,  ami  magnesia-rich 
marbles;  diallage  is  common  in  gabbros  and 
related  rocks,  and  in  jx-ridotites  and  the  serpen- 
tines derived  from  them.  Pyroxene  cleavage 
separates  these  two  minerals  from  all  but  other 
pyroxenes,  from  which  the  extinction  angle 
separates  them.  In  sections  showing  parallel 
extinction,  the  plane  of  the  optic  axes  is  parallel 
to  (010),  in  olivine  it  is  parallel  to  (001). 

Aegirite-augite,  a  pyroxene  of  the  igneous 
rocks  rich  in  sodium,  especially  of  nephelite- 
syenitcs,  phonolites,  leucitophyres,  etc.,  also  of 
some  alkali-granites  and  -syenites,  shows  the 
extinction  angle  of  augite  but  the  peculiar  green 
color  of  aegiritc.  Pleochroism  is  the  same  as  in 
aegirite,  a  =  grass-green,  b  =  light  green,  c  =  yel- 
lowish to  brownish.  Optically  it  is  probably 
positive.  Aegirite  is  negative  and  has  an 
extinction  angle,  c:o  =  about  5°,  augite  has 
different  color  and  slight  pleochroism. 

Orthite  (allanite),  the  cerium  epidote,  occurs 
in  various  granitic  rocks,  in  the  form  of  grains, 
prisms,  and  rods  along  6  or  c,  with  high  (0.032) 
birefringence  in  fresh  material  but  sinking  to 
zero  when  the  mineral  is  altered  to  a  megascopi- 
cally  gumlike  substance.  Cleavage  (001)  is  dis- 
tinct. Zones  not  uncommon.  Pleochroism, 
strong,  in  fresh  material,  o  =  greenish  brown, 
b  =  reddish  brown,  c  =  brownish  yellow.  Brown 
hornblende  has  smaller  extinction  angle, 
and  distinct  cleavage,  r  u  t  i  1  e  and  cassiter- 
i  t  e  are  uniaxial  and  have  higher  birefringences 
anil  indices  of  refraction.  , 

THanolimne  resembles  olivine  in  having  no 
cleavage,  but  it  is  pleochroic  with  o  =  rcd,  b  = 
c= light  yellow.  2V=G2°-{>30.  The  transition  be- 
tween olivine  and  titanolivine  is  usually  gradual. 


28 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


The  Mineral  is  Negative 

Epidote  (pistacite,  green  epidote)  is  a  com- 
mon contact  or  dynamo-metamorphic  mineral 
in  impure  calcareous  rocks,  and  a  secondary 
mineral  in  the  feldspars  of  many  igneous  rocks. 
It  is  often  associated  with  clinozoisite.  It  has 
been  described  as  primary  in  certain  granites. 
Pistacite  is  the  iron-rich  epidote,  clinozoisite  the 
iron-poor  or  iron-free  variety.  The  characteristic 
pistachio  green  color,  brilliant  interference  colors, 
and  high  relief  separate  it  from  all  other  minerals. 
Pleochroism,  a  =  colorless  to  yellowish  or  greenish, 
b  =  yellowish  to  yellowish  gray,  c  =  green  to  light 
yellowish  brown,  sometimes  rather  weak.  The 
plane  of  the  optic  axes  lies  at  right  angles  to  the 
elongation  of  the  crystal. 

Paragonite  is  common  in  certain  schists  and 
probably  also  occurs  as  a  secondary  mineral  from 
plagioclase  (see  under  muscovite).  It  cannot  be 
distinguished  optically  from  muscovite. 

Zinnwaldite  is  the  lithia-iron  mica  between 
lepidolite  and  biotite.  It  is  found  in  greisens 
and  rocks  associated  with  tin  ores.  It  has 
weaker  pleochroism  than  biotite,  c>b>a,  with  c 
and  a  =  dark  brown,  brownish  gray,  b  =  yellowish 
brown  or  reddish,  nearly  colorless.  Orientation 
as  in  biotite:  6  =  b,  c:a=0°  to  +7°,  2E=10°-60°. 
Lepidolite  has  a  different  position  of  the  plane 
of  the  optic  axes  (6  =  c,  c:a  =  0°  to  +2°,  rarely. 
b  =  b,  c:c  =  0°  to  +2°).  It  may  be  impossible  to 
separate  this  mineral  from  biotite  except  by 
the  reaction  for  lithium. 

Biotite  is  a  common  mineral  of  the  acid  and 
intermediate  igneous  rocks.  It  also  occurs  as 
a  metamorphic  mineral  in  gneisses,  schists,  and 
various  other  rocks.  In  many  cases  it  is  inter- 
grown  with  muscovite,  either  in  parallel  position 
or  with  muscovite  forming  the  outer  zone.  Pleo- 
chroism, strong  c  <  b>a;  c  and  b  =  deep  brown  to 
red-brown,  deep  green,  a  =  light  yellow  to  reddish, 
light  greenish.  Has  a  golden  brown  color  in 
some  nephelite-syenites.  Basal  sections  are  non- 
pleochroic  and  give  nearly  or  quite  uniaxial 
figures.  "Bird's-eye  maple"  appearance  is  char- 
acteristic of  all  micas.  Pleochroic  halos  about 
minute  inclusions  of  zircon,  etc.,  are  common 
(Manual,  p.  323).  Tourmaline  is  darkest 
when  its  long  direction  is  at  right  angles  to  the 
polarizer;  lepidolite  is  non-pleochroic; 
zinnwaldite  has  less  pleochroism,  and  occurs 
in  greisens  and  with  tin  deposits,  but  may 
require  test  for  Li  to  distinguish.  P  h  1  o  g  o  - 
p  i  t  e  is  less  pleochroic  and  generally  occurs  in 
crystalline  limestones.  Hornblende  is  bi- 
axial, has  inclined  extinction,  and  does  not  show 
the  "bird's-eye  maple"  appearance. 


The  Mineral  is  Positive 

Olivine,  the  intermediate  magnesia-iron 
variety,  is  a  common  primary  mineral  in  basic 
rocks.  It  also  occurs  as  an  accessory  in  basic 
schists,  gneisses,  and  marbles.  It  alters  to  actino- 
lite,  anthophyllite,  iddingsite,  magnetite,  chromite, 
opal,  quartz,  serpentine,  tremolite,  and  other 
minerals.  Alteration  to  serpentine  and  magne- 
tite are  most  common.  It  gelatinizes  slowly  with 
HC1.  The  plane  of  the  optic  axes  is  parallel  to 
(001)  while  in  pyroxenes,  in  sections  show- 
ing parallel  extinction,  it  is  parallel  to  (010). 
F  a  y  a  1  i  t  e  has  2V  =  50°,  higher  birefringence, 
and  is  negative.  Forsterite  has  a  different 
mode  of  occurrence. 

Astrophyllite,  a  rare  mineral  of  the  nephelite- 
syenites,  has  pleochroism,  a  =  yellow  to  red, 
b  =  orange,  c  =  citron-yellow.  2V  =±  75°,  2E  = 
ca.  160°.  It  occurs  in  plates,  laths  along  b,  leaves, 
and  rosettes.  Cleavage  (010)  perfect.  Micas 
have  smaller  axial  angles  and  different  pleo- 
chroism, and  are  negative.  Lavenite  is 
negative.  Staurolite  has  lower  birefrin- 
gence and  a  different  mode  of  occurrence. 

Titanite,  in  the  form  of  prisms,  rhombs,  and 
grains,  is  a  very  common  mineral  in  acid  plu- 
tonites,  such  as  granites  and  syenites,  abundant  in 
nephelite-syenites,  and  less  common  in  diorites. 
It  is  also  abundant  in  some  schists,  gneisses,  and 
limestones.  As  a  secondary  mineral  (leucoxene) 
it  is  derived  from  titaniferous  minerals.  Pleo- 
chroism weak,  c>b>d,  in  brown  and  yellow 
tones,  c:c=+39°,  and  2E  =  45°-68°.  The  strong 
dispersion  produces  colored  isogyres.  M  o  n  a  - 
z  i  t  e  has  lower  birefringence,  smaller  extinction 
angle,  and  weak  dispersion.  Brookite  has 
parallel  extinction,  2V  =  0°  to  —  23°,  and  r  u  t  i  1  e 
is  uniaxial. 

Brookite  occurs  in  veins  with  various  other 
minerals  such  as  albite,  quartz,  nephelite,  rutile, 
garnets,  etc.,  and  in  gold  washings,  always  in  the 
form  of  crystals.  The  acute  bisectrix  is  normal 
to  (100)  but  the  axial  plane  is  parallel  to  (001)  for 
red  and  yellow,  and  parallel  to  (010)  for  green 
and  blue.  For  red  (670  MM)  2E  =  58°0',  yellow 
(589  MM)  2E  =  38°10',  yellowish  green  (555  MM) 
2E  =  0°,  green  (535-525  MM)  2E  =  21°40'-33°0' 
(Manual,  p.  444,  Figs.  619-23).  The  interfer- 
ence figure  for  white  light  is  a  combination  of  all 
of  these  (Manual,  Fig.  624).  Cassiterite 
and  rutile  have  different  habits,  and  brookite 
has  very  different  strengths  of  double  refraction 
in  (100)  and  (010)  sections. 


OF   ROCK-FORMIN'.     Ml\KHM.->     SSI)    ROCKS  29 

The  Mineral  is  Negative 

I'hlogopiti'  is  paler  brown  than  biotitr,  red- 
dish  brown,  or  yellowish  brown,  sometimes  green- 
ish or  colorless.  L'V,  as  in  biolite,  is  small  to  0°. 
Biotitc  has  stronger  pleochmNm,  but  when 
bleached  may  not  In-  distinguishable  from  it. 
Phlogopite  is  essentially  a  mineral  of  marble- 
ainl  crystalline  dolomites,  but 'does  occur  in  the 
leucite  rocks  of  Wyoming  and  in  the  mica- 
peridotites  of  southern  Illinois.  For  separation 
from  other  minerals,  see  under  muscovite. 

brown  iron  amphibole  of  fibrous, 
leafy,  lamellar,  or  granular  form,  is  a  constituent 
of  metamorphosed  carbonate  rocks  whose  chief 
or  only  carl>onate  is  of  iron.  Pleochroism  c  = 
light  brown,  b  =  a  =  colorless.  Amphibole  cleav- 
parates  it  from  other  minerals,  plcochroism 
and  extinction  angle  (11°-15°)  from  other  amphi- 
boln. 

I'lviuliti.  the  iron  olivine,  may  be  colorless 
or  yellowish,  greenish,  reddish,  with  weak  or 
no  pleochroism  in  yellow  and  red  tones.  Oliv- 
ine has  2V  =  88°  (fayalite,  2V==t50°),  is 
optically  positive,  and  has  lower  birefringence. 
Forsterite  is  positive,  has  2V  =  86°,  lower 
indices,  and  different  occurrence  (contact  mineral 
in  nietaniorphic  limestones).  . 

Aegirile,  a  constituent  of  sodium-rich  igneous 
rocks,  especially  nephelite-syenites,  phonolites, 
and  leucitophyres,  but  also  of  some  granites  and 
syenites,  occurs  in  the  form  of  thin  needles  or 
crystals  bluntly  terminated.  The  pleochroism, 
a  =  deep  green,  b  =  lighter  green  to  yellowish  green, 
c  =  yellowish  to  brownish,  and  the  extinction  angle, 
c:a  =  3°-6°,  separate  it  from  other  pyroxenes;  the 
pyroxene  cleavage  from  other  minerals.  A  c  - 
mite  occurs  in  crystals  with  acute  terminations 
and  is  brownish.  Aegirite-augite  has 
a  large  extinction  angle. 

Acmite  occurs  in  long  prismatic  crystals  with 
characteristic  acute  terminations.  Occurrence 
same  as  aegirite.  Color,  brownish  to  reddish 
brown,  often  zonal  around  green  centers  of 
aegirite.  Pleochroism,  o  =  brown,  b  =  light  brown, 
c  =  greenish  yellow.  Extinction,  c:o  =  3°-6°. 
A  c  g  i  r  i  t  e  has  different  pleochroism,  and  occurs 
in  bluntly  terminated  crystals.  Other  pyrox- 
enes have  little  or  no  pleochroism  and  larger 
extinction  angles. 

Piedmontite,  a  manganese  epidote,  occurs  in 
glaueophane-  and  other  schists,  rarely  in  certain 
porphyries,  for  example,  porfido  rosso  antico. 
Pleochroism,  strong  and  characteristic,  o  =  orange, 
b  =  violet,  amethyst,  c  =  red.  The  epidote-like 
character  of  piedmontite  and  its  characteristic 
pleochroism,  separate  it  from  all  other  minerals. 


30 


ESSENTIALS  FOR  THE  MICROSCOPICAL,  DETERMINATION 


THE  DETERMINATION  OF  THE  FELDSPARS 

The  general  characteristics  of  all  members  of  the  feldspar  group  are  the  same.  They  are  usually 
colorless,  belong  to  the  monoclinic  or  triclinic  systems  (with  close  resemblance  in  angles,  twinning, 
etc.),  have  a  cleavage  of  from  86°  to  90°,  a  hardness  of  from  6.0  to  6.5,  and  a  specific  gravity  of  from 
3.84  in  celsian,  through  2.55  in  orthoclase,  to  2.76  in  anorthite. 

They  may  be  classified  as  follows: 


Monoclinic 

Composition 

Triclinic 

Celsian 
Orthoclase 
Soda  orthoclase 

BaO-Al2O3-2SiO2 
K2O-Al2O3-6SiOj> 
(K,  Na)2OAl2O3-6SiO2 
Na2O-Al2O3-6SiO2 

Microcline 
Anorthoclase 
Albite 

CaO-Al2O3-2SiO2 

The  twinning  is  one  of  the  most  important  characteristics  of  the  feldspars,  and  by  the  different 
extinction  angles,  the  various  members  may  be  distinguished.  The  three  most  important  kinds  of 
twinning  are  Carlsbad,  in  which  the  composition  plane  is  one  parallel  to  the  c  axis,  usually  near 
(010),  and  the  twinning  axis  the  c  axis;  a  1  b  i  t  e  twinning  in  which  the  composition  plane  is  (010) 
and  the  twinning  axis  normal  to  this  face,  and  pericline  twinning  in  which  the  6  axis  is  the  twin- 
ning axis  and  the  composition  plane  is  an  inclined  plane  approximately  parallel  to  the  basal  plane 
though  tilted  backward  in  albite  and  down  in  front  in  anorthite.  All  of  these  kinds  of  twinning  may 
be  combined  in  a  single  crystal,  and  they  may  be  repeated  many  times  to' form  the  so-called  poly- 
synthetic  twinning.  When  albite  and  Carlsbad  twins  are  combined,  the  albite  twinning  may  be 
recognized  upon  the  (001)  face  by  the  fact  that  the  elongation  of  the  twinning  lamellae  lies  in  the 
direction  of  the  faster  ray,  while  in  pericline  twinning  this  length  is  the  direction  of  the  slower  ray. 

Orthoclase  (Fig.  1)  is  negative,  6  =  c,  a:a=  ±5°,  extinction  on  (001)  from  (010)  cleavage  =  0°, 
on  (010)  from  (001)  cleavage  =  ±5°,  2V  =  70°  to  80°,  2E  =  120°  ca.,  dispersion  p>v,  indices  of  refraction 
less  than  Canada  balsam.  The  most  common  form  of  twinning  in  orthoclase  is  on  the  Carlsbad  law. 
The  twins  are  turned  180°  with  respect  to  each  other.  In  the  (001) :  (100)  zone 
the  extinction  is  parallel  to  the  (010)  cleavage  and  to  the  twinning  line.  When 
the  twinning  line  shows  on  the  (010)  face,  it  makes  an  angle  of  63°57'  Q3) 
with  the  (001)  cleavage  and  of  ±21°  with  the  extinction  of  each  individual 
(c:b  =  19°-23°).  In  this  zone,  as  the  sections  depart  from  (010)  and  approach 
(100),  the  angle  of  the  cleavage  with  the  twinning  line  naturally  increases  from 
63°57'  to  90°.  The  extinction  angle  also  changes  from  21°  to  90°,  the  increase 
being  slight  at  first,  but,  as  the  section  approaches  the  (100)  face,  the  change  of 
extinction  angle  becomes  very  rapid.  In  any  section  in  this  zone  the  twinning 
line  bisects  the  extinction  angle  and  the  cleavage. 

The  (001) :  (010)  zone  of  one  individual  of  a  Carlsbad  twin  almost  coincides 
with  the  (T01) :  (010)  of  the  other.     In  all  sections  in  this  zone  the  cleavage  cracks 
of  one  individual  are  parallel  to  the  twinning  line,  and  the  extinction  angle  from  this  line  varies 
from  0°  on  (001)  to  3°  to  7°  (12°  in  soda-orthoclase)  on  (010).     In  the  other  individual  the  cleavage 
lines  are  at  right  angles  to  each  other,  and  the  extinction  is  parallel  to  them  and  to  the  twinning  line. 
As  the  section  approaches  the  (010)  face,  the  extinction  angle  increases  until  it  reaches  ±48°  on  (010). 
Baveno  and  Mannebach  twins  are  less  common.     In  the  former  the  twinning  axis  is  the  line 
normal  to  (021)  which  is  also  the  composition  plane.     In  sections  at  right  angles  to  this  plane,  the 


Fio.  1. — Section 
through  a  crystal 
of  orthoclase,  par- 
allel to  (010). 


"I      l;.n   K-l-'tiltMIM.     MlM. KM.-*     \M>    ll()CK8  31 

twinning  line  is  diagonal  to  the  cleavage.  The  two  parts  extinguish  :it  tin-  -ame  time  and  arc  parallel 
to  the  cleavage,  Imt  the  direction  of  n  in  one  individual  is  at  right  angles  to  a  in  the  other,  and  the 
interference  figures  lie  at  right  angles  t,,  ,.:l,.|,  other.  Manneliach  twinning  is  comparatively  rare. 
The  (Hll  i  plane  i^  the  composition  plane  and  the  twinning  axis  is  a  line  at  right  angles  to  it. 

S  a  n  i  d  i  n  e  is  like  orthoclase  in  all  its  properties  except  that  L'Y  is  much  smaller,  varying  from 
very  small  to  0°.  The  orientation  may  U>  different.  It  is  either  as  in  orthoclase  or  in  some  cases 
6  =  b,  a:a=-f-5°,  in  which  case  the  dispersion  is  p<v,  otherwise  p>v  as  in  orthoclase. 

Microcline,  chemically  like  orthocla.-e,  is  likewise  negative.  The  extinction  angle  on 
(010)  = -f-o°,  on  (001)  = +10°.  Indices  of  refraction,  birefringence,  and  dis|>ersion  like  orthoclase. 
_'\  -71°  to  84°.  Combined  polysynthctic  twinning  on  albitc  and  periclinc  laws  is  almost  universally 
ut,  giving  rise  to  a  plaid  effect  or  so-called  "grating"  texture. 

Anort  hoclase,  negative,  slightly  higher  in  refractive  indices  but  with  the  same  birefrin- 
gence as  orthoclase.  ha-  the  same  dispersion,  2V  =  43°-53°,  extinction  on  (010)  =  +4°  to  +10°,  on 
(Mil)  =  +1°  to  +4°.  In  some  cases  it  is  polysynt helically  twinned  on  the  albite  and  pericline  laws  like 
microcline,  from  which  it  is  then  separated  by  the  extinction  angles  on  (001)  and  (010),  and  by  the 
smaller  optic  axial  angle.  It  is  separated  from  all  plagioclases  but  albite  by  its  low  refractive  indices; 
from  albite  by  its  optically  negative  character. 

Plagioclase  feldspars.  The  plagioolase  feldspars  form  an  isomorphous  series.  In 
this  book  the  divisions  are  made  as  shown  in  the  feldspar  diagrams  inside  the  back  cover.  The 
cleavage  (001)  to  (010)  is  practically  the  same  in  all,  varying  from  86°24'  in  albite  to  85°50'  in  anor- 
thite.  The  angle  d-  between  crystallographic  a  and  c,  is  116°29'  in  albite  and  115°55.5'  in  anorthitc. 

All  of  the  plagioclases  are  found  in  all  kinds  of  igneous  rocks,  both  plutonites  and  extrusives. 
It  is  to  be  noted,  however,  that  only  one  kind  of  plagioclase  of  the  same 
generation  occurs  in  any  igneous  rock.  The  feldspar  of  a  plutonite  may  l>c 
zonal,  the  more  basic  plagioclase  in  the  center,  and  the  zones  progressively  more  acid  toward  the 
(K-riphery,  but  two  independent  crystals  of  different  plagioclases  do  not  occur.  It  is  true  that  a 
section  cut  through  and  parallel  to  the  outer  rone  of  a  banded  plagioclase  may  show  nothing  but 
acid  plagioclase,  for  example,  yet  zonal  growth  in  other  crystals  will  show  that  the  section  was  not 
cut  from  a  crystal  entirely  of  acid  plagioclase.  In  the  extrusive  rocks  the  plagioclases  of  the  pheno- 
crysts  and  those  of  the  groundmass  may  be,  though  they  need  not  be,  different,  but  they  represent  two 
generations;  all  of  the  material  of  each  is  of  its  own  kind.  The  phcnocrysts,  having  crystallized 
first,  are  usually  the  more  basic. 

Besides  occurring  in  igneous  rocks,  albite  occurs  as  a  secondary  mineral  in  gneisses  and  schists, 
and  is  also  found  disseminated  through  certain  limestones.  Anorthitc  has  been  found  in  meteorites 
and  in  slags. 

Plagioclase  is  almost  invariably  twinned  on  the  albite  law  and  shows  varying  antl  characteristic 
extinction  angles  for  each  meinlx>r  of  the  group.  Many  methods  for  their  separation  have  been  pro- 
posed. The  most  useful  of  these  are  given  below  and  are  shown  graphically,  where  possible,  in  the 
back  of  this  book. 

1.  By  specific  granites.— Curve  A.    The  value  of  the  specific  gravity  is  constant  so  long  as  the 
material  used  is  pure  and  unaltered.     Glassy  inclusions  or  alteration  to  kaolin  reduce  the  value,  while 
the  inclusion  of  most  other  minerals  or  the  alteration  to  carbonates,  sericite,  paragonite,  or  saussurite 
increases  it. 

2.  By  the  optical  character  of  the  mineral.— Curve  B.     The  optical  character  alone  does  not  deter- 
mine the  kind  of  plagioclase,  but  it  is  of  value  when  taken  in  connection  with  other  properties.     When 
the  value  of  2V  is  between  85°  and  90°  the  curvature  of  the  isogyre  is  too  slight  to  be  seen,  conse- 
quently the  position  of  the  acute  bisectrix  (which  is  always  on  the  convex  side  of  the  isogyre  when  it 
is  placed  at  45°  to  the  cross-hairs)  cannot  be  determined. 


32  ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 

3.  By  the  relative  indices  of  refraction  of  the  feldspar  and  some  known  mineral  with  which  it  is  in 
contact  (Becke  method). — Curve  C.     When  the  feldspar  lies  in  contact  with  a  known  mineral,  their 
relative  indices  may  be  determined  by  the  movement  of  the  Becke  line.     By  making  use  of  several 
sections,  the  indices  in  different  directions  may  be  determined  (Manual,  pp.  277-83).     The  most 
common  substance  used  for  comparison  is  Canada  balsam  although  its  index  varies  slightly  in  differ- 
ent sections,  depending  upon  age  and  the  original  solvent  or  amount  of  heat  used  in  mounting.     The 
index  in  good  sections  should  lie  between  1.534  and  1.540  (Manual,  pp.  283-85).     In  this  book 
the  Canada  balsam  line  is  shown  at  the  mean,  1.537.     It  is  shown  by  the  broken  line  in  the  figure. 
The  lines  e  and  w,  shown  in  the  same  place,  are  the  indices  of  quartz. 

4.  By  determining  the  refractive  indices  by  immersion  in  various  liquids. — This  method  and  a  list 
of  various  immersion  fluids  are  given  elsewhere  (Manual,  pp.  249-65).     The  method  has  been  very 
extensively  used,  and  recently  Larsen  published  a  complete  list  of  all  minerals  arranged  according 
to  their  refractive  indices  (Bull.  679,  U.S.  Geol.  Survey,  1921).     Tsuboi,  in  a  paper  published  in  the 
Japanese  language  (Jour.  Geol.  Soc.  Tokyo,  Vol.  XXVII,  1920),  used  the  method  in  connection  with 
cleavage  flakes  of  feldspar,  and  gives  the  curves  shown  in  D.     These  are  of  much  greater  practical 
value  than  those  giving  a,  /3,  and  7. 

5.  By  extinction  angles  on  cleavage  flakes  parallel  to  (010). — Curve  E.     These  values  were  deter- 
mined by  Schuster  (Tscherm.  Min.  Petr.  Mitt.,  Ill  [1880],  117).     He  considered  extinction  angles 
measured  clockwise  from  cleavage  on  (010)  and  (001)  as  positive,  and  counter-clockwise  as  negative. 
The  (010)  face  may  be  recognized  by  the  fact  that  albite  twinning  lamellae  are  wanting,  although 
those  according  to  the  pericline  law  are  occasionally  seen.     The  crystal  form  is  often  shown  in  outline 
or  by  zonal  growth.     The  (001)  cleavage  is  usually  distinct,  and  is  best  seen  when  the  diaphragm 
below  the  stage  of  the  microscope  is  partially  closed.     In  the  acid  plagioclases  the  elongation,  as 
defined  by  cleavage,  is  nearly  parallel  to  a.     The  extinction  is  measured  from  the  (001)  cleavage. 

6.  By  extinction  angles  on  cleavage  flakes  parallel  to  (001). — Curve  F.     These  values  were  also 
determined  by  Schuster.     Plates  on  (001)  cannot  be  recognized  in  random  fragments  in  rock  sections, 
but  must  be  obtained  by  crushing,  not  grinding,  a  fragment  of  the  feldspar.     Breaking  along  the 
cleavage,  many  of  the  flakes  will  be  found  to  be  parallel  to  (010)  or  (001).     Only  flakes  of  less  than 
0.5  mm.  in  thickness  and  with  parallel  faces  (which  may  be  recognized  by  their  uniform  interference 
colors)  are  of  use.     The  (001)  flakes  show  albite  twinning,  while  those  parallel  to  (010)  do  not.     In 
the  (001)  sections  the  extinction  is  measured  from  the  twinning  lamellae. 

7.  By  the  position  of  the  bisectrix  in  (010)  plates. — (Becke,  Tscherm.  Min.  Petr.  Mitt.,  XIV  [1894], 
375,  415;   XVI  [1897],  180.)     In  convergent  light  the  different  plagioclases  show  different  positions 
of  emergence  of  the  bisectrix  in  (010)  plates.     In  air  they  are  as  follows  when  the  flake  is  oriented  with 
the  c  axis  vertical  and  the  (001)  face  sloping  from  southwest  to  northeast: 

In  albite  the  inclination  is  small  and  the  positive  bisectrix  (c)  emerges  below  the  center  and 
slightly  to  the  right.  Inoligoclase  the  bisectrix  is  nearly  normal  to  the  face  but  slightly  above 
the  center  and  to  the  left.  In  andesine  the  bisectrix  lies  nearly  20°  to  the  top  and  left.  In 
labradorite,  upon  the  left  face,  the  axis  is  not  in  the  field;  only  part  of  a  bar,  part  of  one  system 
of  axial  rings,  and  a  small  part  of  the  lemniscate  curves  appear.  The  bisectrix  lies  off  the  field  above 
and  to  the  right.  Inbytownite  the  figure  is  similar  to  that  in  labradorite  except  that  no  lemnis- 
cate curves  appear,  only  the  circular  rings  about  the  melatope,  which  is  off  the  stage  but  near  the 
edge  of  the  field  in  the  southwest,  are  seen.  The  bisectrix  lies  to  the  northeast.  In  anorthite 
the  melatope  appears  at  the  edge  of  the  field  at  the  southwest. 

8.  By  the  extinction  angles  on  sections  cut  at  right  angles  to  both  (001)  and  (010). — This  method  was 
used  by  Becker  (Eighteenth  Ann.  Rept.  U.S.  Geol.  Survey,  III  [1898],  34)  and  by  Becke  (Tscherm.  Min. 
Petr.  Mitt.,  XVIII  [1900],  556).     Curve  G.     These  sections  are  easily  recognizable  in  microlites  and 
in  phenocrysts  of  extrusive  rocks  by  their  nearly  quadratic  sections,  and  in  plutonites  by  zonal  growths 


OK  KiK-k-FoRMiNc  MINERALS  AND  ROCKS  33 

with  quadratic  outlines.  Sections  :it  right  angles  to  both  ltd!  i  and  (010)  have  tin-  division  lines 
between  the  albite  twinning  lamellae  and  tin-  i(M)l)  cleavage  lines  extending  at  right  angles  to  the 
section:  consequently  when  the  tul>e  of  the  microscope  is  slightly  raised  or  lowered,  there  will  be 
no  lateral  displacement  of  these  lines.  The  small  cross-sections,  shown  at  the  side  of  the  curve, 
indicate  the  directions  of  i  -f  i  and  i  -  )  extinction  to  a. 

If  the  cross-section  does  not  happen  to  be  exactly  at  right  angles  to  (001)  and  (010)  it  does  not 
greatly  matter,  for  the  variation  on  tilting  the  section  slightly  is  not  great.  This  method  is  good 
where  applicable,  because  the  increase  in  the  extinction  angles  from  albite  to  anorthitc  is  rapid  and 
uniform. 

!i.  />'//  tin  ijrtincliiin  angles  on  sections  from  the  (001)  (010)  zone. — (Extinction  angles  of  micro- 
htes,  after  Wiilfing,  Mikroxkop.  Physiog.,  I1,  361-62.)  Curve  H.  Spherulite  rays  and  the  micro- 
lites  of  the  extrusive  rocks  are  Ixmnded  by  these  cleavages.  The  curve  alx>ve  12°  is  good  if  albite 
and  andesine  are  separated  by  their  refractive  indices. 

10.  By  the  <-xtincti<>n  nmjles  on  sections  at  right  angles  to  the  optic  normal  (b).     Curve  I.     This 
is  the  method  <>f  Fedorow.     The  interference  colors  between  crossed  nicols  of  sections  at  right  angles 
to  the  optic  normal  are  the  highest  of  any  in  that  mineral,  though  rarely  exceeding  pale  yellow  in 
normal  sections  of  feldspar.     In  the  acid  plagioclases,  the  extinction  angles  vary  only  from  +2°  to 
—  2°,  but  from  andesine  to  anorthite  they  change  rapidly  and  may  be  used. 

11.  liy  the  extinction  angles  on  sections  at  right  angles  to  either  bisectrix,  as  used  by  Fouqu6  (Bull.  Soc. 

\\  II  [1894],  306).  Sections  cut  at  right  angles  to  either  bisectrix  may  l>e  recogni/ed 
l>y  their  intermediate  birefringence.  In  convergent  light  the  interference  figure  will  close  as  a  cross 
in  the  center  of  the  field  when  the  principal  sections  of  the  slide  and  nicols  are  parallel.  Rotate  the 
section  to  the  diagonal  position  and  test  by  the  gypsum  plate  whether  it  is  at  right  angles  to  the  a  or 
the  c  axis  (negative  or  positive,  disregarding  whether  the  acute  or  the  obtuse  bisectrix  appears  in  the 
field  i.  In  sections  at  right  angles  to  a  the  extinction  angle  is  measured  from  the  twinning  lines  (solid 
line,  Curve  K).  In  sections  at  right  angles  to  c,  in  the  basic  feldspars,  the  extinction  angle  is  measured 
from  the  trace  of  the  twinning  lines  or  of  the  (010)  cleavage  (Curve  M,  dotted  line).  In  the  acid 
feldspars  the  section  at  right  angles  to  c  is  very  near  the  (010)  face,  and  therefore  shows  neither  twin- 
ning lamellae  nor  (010)  cleavage;  the  extinction  is  measured  from  the  (001)  cleavage  (Curve  L, 
broken  line). 

Sections  at  right  angles  to  a  give  good  values  as  high  as  AbiAni,  and  are  of  use  when  one  can 
determine  the  positive  or  negative  directions  of  extinction.  The  values  in  sections  at  right  angles  to  c 
are  good  in  the  basic  feldspars. 

12.  By  the  extinction  angles  on  sections  from  the  zone  at  right  angles  to  (010)  or  the  symmetrical  zone  — 
(Statistical  method  of  Michel-Levy,  Ann.  d.  Mines,  1877,  pp.  392-471.)     Curve  N.    Sections  in  this 
zone  may  be  recognized  by  the  fact  that  (1)  albite-t  winning  lamellae"  are  separated  by  very  sharp 
lines  which  are  not  laterally  displaced  when  the  microscope  is  focussed,  (2)  the  extinction  angle  from 
the  twinning  line  is  the  same  on  each  side,  (3)  if  the  section  is  turned  to  the  45°  position  the  two 
systems  of  twins  become  of  uniform  interference  color. 

This  method  is  one  of  the  most  valuable  for  the  determination  of  the  feldspars  although  some 
confusion  may  arise  from  the  positive  and  negative  directions  of  extinction  in  albite  on  the  one  hand 
and  andesine  on  the  other.  The  acid  end,  however,  to  about  Ab^Aiiu,  in  sections  in  this  zone,  has 
refractive  indices  less  tharr  Canada  balsam,  so  the  area  of  confusion  falls  entirely  within  oligoclasc. 

It  is  not  necessary  that  the  sections  used  be  absolutely  in  the  symmetrical  zone,  for  there  is  but 
slight  error  if  they  vary  no  more  than  10°  or  15°  from  the  true  i>osition.  In  such  sections  the  extinc- 
tion angles  on  either  side  of  the  twinning  lamellae  will  not  be  the  same,  but  half  of  the  sum  of  the 
two  angles  very  nearly  coincides  with  the  true  values  found  in  the  zone.  For  determination,  there- 
fore, read  the  extinction  angle  of  one  twin  at  one  side  of  the  vertical  cross-hair,  turn  the  section  past 


34 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


the  vertical  cross-hair  to  the  position  of  extinction  of  the  other  twin  on  the  other  side,  and  divide 
the  angle  thus  obtained  by  two. 

From  albite  to  bytownite  (extinction  45°),  the  extinction  angles  are  read  from  the  twinning 
lamellae  to  the  intermediate  vibration  direction,  but  since  a  is  nearly  normal  to  the  section,  the  third 
direction  is  near  c,  consequently  the  measured  angle  is  toward  the  faster  ray.  For  a  short  distance 
beyond  the  point  where  the  extinction  angle  is  45°,  the  angle  is  read  toward  the  other  and  slower 
axis  of  vibration.  In  anorthite,  however,  the  nearest  vibration  direction  is  again  the  faster  way. 

13.  By  the  extinction  angles  on  sections  from  the  zone  at  right  angles  to  (010),  when  the  albite  twinning 
is  combined  with  Carlsbad  twinning. — (Michel-Levy,  Etude  sur  la  determ.  d.  felds.,  Troisieme  fasc., 
1904.)  Occasionally  the  albite  twinning  is  combined  with  Carlsbad  twinning  in  the  same  section. 
In  such  cases  the  combined  extinction  angles  are  characteristic,  without  considering  the  direction 
of  rotation,  and  it  is  not  necessary  to  search  for  the  maximum  angle.  A  section  is  chosen  which  has 
approximately  symmetrical  extinction  in  the  albite  twins  in  each  half  of  the  Carlsbad  twin.  The 
angles  are  measured  in  each  half  as  in  the  preceding  method  and  divided  by  two.  The  angle  of  the 
smaller  pair  is  found  in  the  column  to  the  left  in  Figure  23,  the  larger  on  the  curves.  The  vertical 
line  at  the  intersection  gives  the  feldspar.  In  the  figure  the  broken  lines  indicate  the  angles  which  the 
sections  make  with  the  (100)  face.  The  size  of  this  angle  is  indicated  in  the  diagram  by  the  figures 
which  are  not  followed  by  the  degree  (°)  mark. 


CO' 


20' 


20' 


Fio.  2. — Maximum  extinction  angles  in  the  Pyroxene  and  Amphibole  groups.    Solid  lines  indicate  extinction 
angles  from  c  toT]  broken  lines  from  c  toj( 


OF    KnrK-I  "KMIV.     \llM  .KM..-     \\l> 


88 


PYROXENES  AND  AMPHIBOLES 

Pyroxenes  differ  from  amphilioles  in  having  a  prismatic  angle  of  87°  (amphiboles  124°),  and 
!<•>-  prrfn-t  cleavage.  '1'ln-  crystals  arc  usually  stouter,  the  extinetioii  angles  are  greater, 
ami  ]ilrorhn>ism  is  generally  weaker,  often  wanting  except  in  aegirite-augite,  aegirile.  and  acmite. 
Augite,  also,  is  sometimes  quite  strongly  pleoehroie  in  purple  tones,  and  hypersthene  in  pink  to  green 
tones.  The  monoelinie  pyroxenes  are  separated  from  the  orthorhombic  by  having  inclined  extinc- 
tion. Acmitr,  aegirite,  and  pectolite  have  extinction  angles  usually  less  than  5°,  and  may  be  con- 
fused with  orthorhombic  pyroxenes,  rxcrpt  for  their  pleochrnism.  Upsides  differing  in  pleochroism, 
aegirite  may  l>e  separated  from  orthorhombic  pyroxenes  by  its  much  higher  double  refraction,  and 
its  negative  elongation  (all  orthorhombic  pyroxenes  have  positive  elongation).  Basal  sections  of 
orthorhomliir  sections  in  convergent  light  show  the  emergence  of  a  positive  bisectrix  in  the  center 
of  the  field,  monoelinie  pyroxenes  which  have  low  extinction  angles  show  a  negative  bisectrix,  while 
the  other  monoelinie  pyroxmrs  show  the  emergence  of  an  axis.  The  chief  mode  of  separation  of  the 
pyroxenes  from  each  other  is  by  means  of  extinction  angles  as  shown  by  Figure  2.  All  pyroxenes 
are  separated  from  other  minerals  by  their  characteristic  cleavage. 


PYROXENES 


Name 

tjM  m 

Mtal 

I  'll.ir 

acter 

Orientation 

Optic  Angle 

BlreMn- 
gvnce 

Pleochrolon 

Enstatite  ... 
Hnmzitc.  . 
Hypersthene  ... 

1  iin|)-ii|i'   .  .  . 

Ortho. 
Ortho. 

Ortho. 

Mono. 
Mono. 
Mono. 
Mono. 
Mono. 
Mono. 
Mono. 
Mono. 
Mono. 
Mono. 
Mono. 

+ 
+ 

+ 
+ 
+ 
+ 
+ 

- 
+ 
+ 

+ 

e—  C 
c-C 
C—C 

c:C--39° 
c:c--39° 
c:e--44° 
c:c--45°to  -55° 
c:t--55°  to  -87° 
c:tt--3°    to  -6° 
c:a--3°    to  -6° 
c:c--23°  to  -26* 
c:C--33.5° 
c:a-+32° 
c:o--5° 

2E-135* 
2E-1060 
2E-850 

2V  -59° 
2V  -59°  and  less 
2V  -60° 
2V  -60° 

0.009 
0.009 
0.013 

0.029 
0.029 
0.015 
0.025 
0.029 
0.050 
0.050 
0.016 
0.029 
0.015 
0.038 

None 
Faint 
Pink,  green 

None 
None 
Weak,  green 
May  be  purple 
Green 
Green 
Brown 
None 
None 
Nonr 

None 

DialloKt-  
nnl<-iiU-rj!il«- 
Aunitr 

AegiriteHuigite 
AegJrite'  
Acmite  
SpodumeiK-            
Jadeitc.  .  . 
\Vollastoitilt- 

2V  -62° 

2V  -54°  to  60° 
2V  -72° 
2V-40°to69e 
2V  =  60° 

PectoliU- 

AMPHI  HOLES 


Name 

Synutn 

Optical 
CTiar- 
actcr 

Orientation 

Optic  Angle 

Birefrin- 
gence 

I'kxjchrolwn 

Anthophyllite      ... 

Ortho. 

rii 

e—  e 

2V-  -90° 

0.024 

Yellow,  brown,  green 

GtHlriU-         

Ortho. 



C-t 

2V-5r-79° 

0.021 

Yellow,  brown,  green 

Tremolite  ...          

Mono. 

c:c--16° 

2V  -87.5° 

0.026 

Non-pleochroic 

Aotinolite  
(iriinorite  .... 

Mono. 
Mono. 

— 

c:c--15° 
c:e--lleto  -15° 

2V  -80° 
2V  -82° 

0.027 
0.045 

Faint  green 
Colorless,  brownish 

Common  hornblende  .  . 
Piirgnsite  
Katophorite  
Uasaltir  homblemlr 
Karkcvikite.  ... 
Glaucouhane  .  . 

Mono. 
Mono. 

Mono. 
Mono. 
Mono. 
Mono. 

* 
+ 
+ 

c:c--12°to  -20° 
c:e--18°to  -21° 
c:t--23°to  -60° 
c:C-     0°    to  -12° 
c:c--14° 
c:c--4°    to  -6° 

,.,_       R° 

2V  -54°  to  84° 
2V  =  52°  to  60° 
2V  -small 
2V  -80° 
2V  -54° 
2V-50° 
2E=-70° 

0.016 
0.019 
low 
0.021 
0.021 
.   0.018 

Strong  green,  yellowish 
Green,  yellow 
Red,  yellow 
Strong,  green,  brown 
Brown 
Blue,  violet,  greonLsh 
Blue,  violet,  greenish 

Kirlx-rkite 

Mono. 

A 

c:c--85° 

2V  -large 

0.005 

Mine,  yellowish  Kn-en 

Arf  vedsonite  .  .  . 

Mono. 

*• 

^ML_  +  in»t«  +20°j 

2V  -large 

0.021 

Blue  to  greenish 

36  ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 

MODES  OF  OCCURRENCE  OF  VARIOUS  MINERALS 

Minerals  which  occur  in  needle-like  crystals. — Actinolite,  aegirite,  apatite,  aragonite,  cancrinite, 
datolite,  dumortierite,  hydromagnesite,  hydronephelite,  natrolite,  pectolite,  sillimanite,  stilbite, 
tremolite,  topaz,  tourmaline,  wollastonite. 

Minerals  which  occur  in  fibrous  aggregates. — -Chalcedony,  datolite,  gypsum,  hydrargillite,  kaolin, 
natrolite,  prehnite,  sericite,  serpentine,  sillimanite,  talc. 

Minerals  which  occur  in  radiating  groups  of  fibers,  or  as  spherulites. — Brucite,  chlorite,  chalcedony, 
delessite,  natrolite,  pectolite,  quartz-orthoclase,  stilbite,  thomsonite,  other  zeolites. 

Minerals  which  occur  as  cavity  and  interspace  fillings. — Analcite,  carbonates,  chalcedony,  quartz, 
sodalite(?),  zeolites.. 

Minerals  soluble  in  HCl  without  gelatinization. — Brucite,  epistilbite,  hematite  (hot  cone.),  limonite 
(cone.),  magnetite  (cone.),  monazite  (cone.),  pyrrhotite  (cone.),  stilbite. 

Minerals  which  gelatinize  with  HCl.- — Analcite,  anorthite,  cancrinite,  chlorite,  datolite,  fayalite, 
glass,  haiiynite,  hydronephelite,  kaolin,  laumontite,  lazurite,  leucite  (part),  melilite,  nephelite,  nose- 
lite,  olivine,  scapolite  (Ca  end),  sodalite,  wollastonite,  zeolites. 

Effervesce  with  HCl. — Calcite,  cancrinite  (slightly),  dolomite  (hot),  hydromagnesite,  magnesite 
(hot),  siderite  (hot). 

Secondary  minerals.- — Albite  (in  metamorphic  rocks),  analcite,  antigorite,  brucite,  calcite,  can- 
crinite, chlorite,  clinochlore,  epidote,  hematite,  kaolin,  leucoxene  (titanite,  etc.),  limonite,  magnetite, 
opal,  paragonite,  pennine,  pyrite,  quartz,  rutile,  saussurite,  sericite,  serpentine,  talc,  titanite,  uralite 
(amphibole),  zeolites. 

Pneumatolytic  minerals  (frequently  associated). — Fluorite,  lepidolite,  muscovite,  topaz,  tourma- 
line, zinnwaldite. 

Metamorphic  minerals  by  assimilation  of  other  material  in  argillaceous  rocks. — Almandite,  andalu- 
site,  biotite,  cordierite,  corundum,  disthene,  ottrelite,  pleonast,  sillimanite,  spinel,  staurolite. 

Metamorphic  minerals  by  assimilation  in  calcareous  argillites  or  argillaceous  limestones. — Epidote, 
grossular,  scapolites,  vesuvianite,  zoisite. 

Metamorphic  minerals  by  assimilation  in  calcareous  rocks. — Calcite,  datolite(?)  wollastonite. 

Metamorphic  minerals  by  assimilation  in  magnesian  rocks. — Actinolite,  anthophyllite,  brucite, 
diopside,  forsterite,  periclase,  serpentine(?),  talc(?),  tremolite. 

Other  metamorphic  minerals.- — Graphite,  magnetite,  pyrite. 

Minerals  never  secondary. — Apatite,  cassiterite,  eucolite,  eudialyte,  fluorite,  haiiynite,  leucite, 
melilite,  nephelite,  noselite,  pyrope,  sodalite,  tridymite,  zircon. 

Minerals  never  primary. — Chlorite,  serpentine. 

Never  occur  in  igneous  rock^- Anhydrite,  disthene,  gypsum,  ottrelite. 

Rare  yellow  minerals  of  allmi-syenites  and  nephelite-syenites. — Astrophyllite,  hjortdahlite,  laaven- 
ite,  lamprophyllite,  mosandrite,  rinkite,  rosenbuschite.  They  are  always  accompanied  by  fluorite. 

Minerals  which  usually  give  abnormal  Berlin  blue  interference  colors. — Chlorite,  clinozoisite,  melilite, 
vesuvianite,  zoisite. 

Alteration  products  which  occur  in  minute  shreds. — Kaolin,  talc,  white  mica  (paragonite,  sericite). 

Alteration  products  which  occur  in  grains. — Albite,  calcite,  epidote,  leucoxene,  quartz,  saussurite, 
titanite,  zoisite. 

Minerals  which  never  occur  together. — Quartz  with  nephelite,  leucite,  sodalite,  haiiynite,  noselite, 
melilite,  etc.  Primary  muscovite  with  pyroxene.  Two  different  plagioclases  of  the  same  generation 
(see  p.  31).  Quartz  is  rare  with  olivine  except  in  a  few  basalts. 

Significant  mineral  association. — If  aegirite,  acmite,  or  golden  brown  biotite  is  present,  you  will 
frequently  find  nephelite,  leucite,  sodalite,  analcite,  etc.  If  one  pneumatolytic  mineral  is  present 
there  are  usually  others  also. 


OK    ROCK-FORMINC    MlNKUAI.S    AND    HoCKS  37 

A  SUMMARY  OF  PETROGRAPHIC  METHODS 

In  the  fr\v  pap-  following,  reference  can  only  l>e  made  to  those  points  in  the  manipulation  of  the 
microscope  which  the  occasional  worker  constantly  needs  and  is  likely  to  have  forgotten. 

Kjrniniiiiilion  l>i/  nnlinnnj  liijht. — By  ordinary  light  is  meant  light  which  is  not  polarized.  As  11 
matter  of  fact  one  uses  plane-polari/ed  light  for  practically  all  examinations  which  could  also  be  made 
liy  ordinary  light.  Pleochroism  only  is  affected  (229-321)- 

Without  the  analy/.er,  therefore,  the  microscope  is  tested  and  adjusted,  and  the  vibration  directions 
ate  determined  in  the  accessories  (229-32). 

Tn  dtli-rmini  the  ribration  direction  of  the  glow  ray  in  the  oceeMoriet. — Obtain  an  interference  figure  in  the  mica  plate. 

The  line  ronnrrtiiig  tin'  nielatoprs  is  tin-  itirrrtinn  nf  c.  I'lace  the  gy|>»uiii  plat<?  on  the  stage  at  45°  off  extinction,  and, 
with  nicols  crossed,  insert  the  mica  plate  alxive  it.  If  the  interference  color  rises,  the  vibration  rays  of  the  two  are 
parallel;  if  it  falls,  they  are  at  rinht  angles.  Proceed  in  the  same  way  with  the  quurtz  wedge. 

Other  determinations  arc  crystal  form  (233),  cleavage  and  parting  (235),  color  (309-lllT,  and 
refractive  indices  (237-85). 

/;.  r/,v  lim-.  On  raising  the  tube  of  the  microscope,  the  Becke  line  moves  into  the  medium 
having  the  higher  index. 

KjrinniiHitionx  hi/  plane  polarized  light. — In  these  examinations  the  analyzer  (lower  nicol)  is  in 
place,  luit  the  upper  nicol  is  removed.  Here  are  determined  pleochroism  (320-26)  and  refractive 
indices  in  different  directions  in  a  single  mineral  grain. 

Drtrrmitmtinn  of  Ihr  rihraium  direction  in  the  lower  nicol  (178). — A  section  of  biotite  cut  at  right  angles  to  the  cleavage 
has  its  greatest  absorption,  consequently  is  darkest,  when  its  cleavage  direction  is  parallel  to  the  plane  of  vibration  of 
tlie  [H.lari/er.  Tourmaline,  on  the  other  hand,  extinguishes  light  when  its  long  direction  (c)  is  at  right  angles  to  this 

direction. 

Kin  mi  nations  between  crossed  nicols. — When  a  mineral  is  dark  between  crossed  nicols  it  is  iso- 
tropic  (336),  and  may  be  a  section  of  an  isotropic  mineral  (isometric  crystal  or  amorphous  sub- 
stance), or 'an  isotropic  section  of  an  anisotropic  mineral,  that  is,  a  section  cut  at  right  angles  to  an 
optic  axis. 

Extinction  angles  are  measured  between  crossed  nicols  (339,  390-412).  Crystals  of 
the  tetragonal,  hexagonal,  and  orthorhombic  systems  show  parallel  extinction,  that  is,  the  principal 
vibration  directions  are  parallel  to  the  crystallographic  axes. 

It  must  be  rememlxTcd  that  minerals  of  these  systems  which  show  prismatic  cleavage  may  show  a  development  of 
but  one  set  of  cleavage  lines,  for  sections  steeply  inclined  to  the  basal  section  (which  shows  two  set*  of  cleavage  lines) 
will  show  these  lines  forming  an  acute  angle.  If  now  the  section  is  cut  so  that  one  set  of  lines  in  nearly  at  right  angles 
to  the  section,  the  other  will  l>e  nearly  parallel  to  it,  consequently  in  grinding,  the  mineral  will  lx>  separated  more  or  less 
along  the  former  but  not  along  the  latter,  with  the  result  that  a  single  set  of  lines,  inclined  to  the  extinction,  appear. 
Usually  at  the  same  angle  on  the  other  side  of  the  extinction,  traces  of  the  corMppnding  cleavage  may  be  seen  in  the 
form  of  rough  cracks  or  broken  lines. 

Furthermore,  in  minerals  of  these  systems  with  prismatic  cleavage,  the  traces  of  this  cleavage  are  parallel  or  sym- 
metrical with  respect  to  crystallographic  axes  only  in  the  zones  at  right  angles  to  the  three  principal  optic  sections,  that 
is,  at  right  angles  to  the  three  planes  containing  the  crystaUographic  axes.  Interference  figures  on  the  sections  in  two 
of  these  (ones  will  show  a  bar  passing  directly  through  the  center  of  the  field  when  the  bar  is  horizontal  or  vertical, 
consequently  they  may  l)e  so  identified.  In  other  .sections  the  bar  when  vertical  or  horizontal  will  lie  off  to  one  side. 
In  all  sections  which  cut  the  three  crystallographic  axes,  the  extinction  will  lie  inclined  with  respect  to  the 
cleavage,  the  term  parallel  extinction  referring,  of  course,  to  parallelism  with  the  crystallographic  axes  and  not 
with  the  traces  of  cleavage  planes. 

Another  determination  made  between  crossed  nicols  is  birefringence  (348-59,  369-88). 
The  interference  color  produced  by  a  mineral  depends  upon  two  factors,  thickness  of  section  and 
difference  between  the  refractive  indices  in  two  directions. 


1  The  numbers  in  this  section  refer  to  pages  in  the  writer's  Manual  of  Petrographic  Method*,  2d  ed.,  New  York,  1918. 


38 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


This  is  brought  out  by  the  Michel-Le'vy  color  chart  shown  in  Figure  3.  The  numbers  beneath  the  figure  represent 
M(ni—ni)  in  millionths  of  millimeters.  The  ordinates  represent  thickness  of  section.  The  value  of  unit  birefringence, 
iii— nt  (that  is  7  — a  or  u  —  t)  remains  constant  for  any  mineral,  but  as  the  section  increases  in  thickness  so  does  the 
retardation  increase.  The  diagonal  lines  in  the  diagram  represent,  therefore,  the  retardation  produced  by  sections  of 
different  thicknesses  (371-72).  Of  course  the  interference  color  in  a  mineral  depends  also  upon  the  orientation  of  the 
section,  being  zero  or  nearly  zero  along  the  optic  axis  and  increasing  to  a  maximum  in  the  plane  of  the  optic  axes.  To 
measure  the  maximum  birefringence  of  a  mineral,  therefore,  choose  the  section  which  gives  the 
highest  interference  colors. 


„.«» 


FIG.  3. — Outline  of  Michel-Levy's  chart  of  birefringences;  the  positions  of  the  colors  modified  according  to  the 
Kraft  scale  for  a  clear  sky. 

The  optical   elongation  is  often  useful  in  the  separation  of  minerals  (361). 

Place  the  unknown  mineral  on  the  stage  so  that  its  vibration  directions  make  angles  of  45°  with  the  vibration  direc- 
tions in  the  nicol  prisms.  The  light  is  then  at  its  maximum.  Place  above  it,  in  the  slot  provided  for  the  purpose,  the 
gypsum  plate  (Red  of  the  first  order)  or  the  quartz  wedge,  the  former  for  low  colors,  the  latter  for  high.  If  the  inter- 
ference color  increases,  the  vibration  directions  are  parallel,  if  it  decreases,  they  are  at  right  angles.  It  is  usually  well 
to  determine  the  colors  with  the  mineral  successively  in  two  positions  at  right  angles  to  each  other.  From  the  known 
vibration  directions  in  the  accessory,  it  is  thus  determined  whether  the  long  direction  (elongation)  of  the  mineral  is 
parallel  to  the  fast  or  slow  ray.  If  the  elongation  of  the  mineral  is  parallel  to  the  direc- 
tion of  c,  the  mineral  is  said  to  have  positive  (+)  elongation;  if  parallel  to 
the  fast  ray,  n  e  g  a  t  i  ve  (—)• 

Examination  between  crossed  nicols  by  convergent  light. — By  converting  the  microscope  into  a 
conoscope  (413),  it  is  possible  to  determine  whether  a  mineral  is  isotropic,  uniaxial,  or  biaxial.  If  it  is 
isotropic  (415)  no  interference  figure  is  produced. 

The  student  should  familiarize  himself  with  the  interference  figure  produced  by  the  microscope  when  set  up  properly 
as  a  conoscope  but  with  only  a  blank  object  glass  on  the  stage.  All  objectives  give  more  or  less  well-defined  uniaxial 
interference  figures  due  to  polarization  by  the  glass  of  the  objective  or  condenser  (415).  This  should  not  be  confused 
with  the  figure  produced  by  uniaxial  minerals. 

In  crystals  of  the  tetragonal  and  hexagonal  systems,  a  cross  with  a  greater  or  smaller  number 
of  colored  rings  is  produced  (416-19,  425-26). 

The  section  which  will  give  the  best  interference  figure  with  the  cross  in  the  center  of  the  field  is  one  which  is  com- 
pletely isotropic  between  crossed  nicols.  If  no  such  section  can  be  found,  choose  the  one  giving  the  lowest  inter- 
ference colors.  If  the  center  of  the  cross  lies  only  a  little  beyond  the  field  of  view,  the  fact  that  the  mineral  is 
uniaxial,  or  nearly  so,  is  shown  by  the  appearance  of  the  bars  which  remain  parallel  to  the  cross-hairs  during  their  passage 


or  HIM  k- 


MISKKALS  AND  ROCKS 


across  the  stage     The  greater  th.-  di-t  inn-  fri>m  th.  . ,  nt.  r  of  the  -ta«e  to  the  center  of  the  cross,  the  greater  the  flora 

of  the  l>:ir     ll.s,  Im   ."..".I        'I'liere  is  thus  :in  iipp:irent  curvature. 

Simie  idea  uf  the  strength  <>f  the  birefringence  may  IK'  iil)t:iiinil  I iy  tin-  number  of  rings  in  the  interference  figure 
'j:ti.      This  is  .sometimes  of  value.  i-s|»-i«i;(lly  in  colored  minerals  whose  interference  colors  an-  liiililen  by   the  color 

of  the  mineral      ljuart/..  in  a  section  of  normal  thicknes-    (MU.'inim  .  with  e—u  — 0.009,  iihoWH  a  black  crntui  anil  the 

first  yellow  rin»i  at  the  |>eriphery  of  the  Held  of  \  i<  »       Hiotite  with  y—  O-0.040,  give*  two  colored  rings,  and  calcite, 

with  ui  — t  =  0.17-,  give-  i, MI  many  to  count. 

In  crystals  of  the  ortliorhoinbic,  monoclinic,  and  triclinic  systems,  the  interference  figures  are 
( I'.'O  L'l.  I  •_'•'.        I'nless  tin-  optic  axial  angle  is  very  small,  only  a  portion  of  the  figure  can  l>e 


r'li;.    I      Quartz    (-(-)    under   a 

.m  plate. 


.">. — A  positive  uniaxial  in- 
terference figure.  The  arrow.s  inili- 
cate  the  movement  taking  place 
upon  inserting  a  quartz  wedge 
al lovi-  the  gcetion. 


«.—  (jinirlz  (  +  ),  with  the 
center  of  the  cross  outside  the  tieM 
of  the  micro.sco|x>,  an  seen  under  a 

plate. 


.-< -i -n.     The  average  microscope  will  show  the  two  melatopes  of  topaz  just  at  the  limits  of  the  field 

(2E=130°ca.). 

l'"r  the  determination  of  the  uniaxial  or  biaxial  character,  a  figure  with  both  bars  off  the  stage  may  suffice,  but  to 
ileterinine  the  optical  character,  it  is  necessary  (unless  the  optic  axis  be  measured)  that  a  melatope 
remain  in  the  field  of  view  during  a  complete  rotation  of  the  stage,  for  I  he  melittopc 
is  at  its  greatest  distance  from  the  center  in  the  4.r>°  position,  which  is  the  position  in  which  the  curvature  of  the  bar  is 
to  be  determined.  In  the  45°  position  the  convex  sideof  the  bar  is  toward  the  acute 
h  i  >  e  c  t  r  i  x  .  For  the  determination  of  the  optical  character,  therefore,  it  is  not  the  most  symmetrical  figure  (at 
right  angles  to  a  bisectrix)  which  is  best,  but  the  one  most  nearly  at  right 
angles  to  an  optic  axis.  Since  light  is  dispersed  in  all  biaxial  crystals, 
such  a  section  can  be  actually  at  right  angles  to  an  optic  axis  only  for 
a  given  color,  consequently  is  never  actually  isotropic.  As  in  uniaxial 
crystals,  so  here  also,  the  section  giving  the  lowest  color  gives  the  best 
figure  for  the  determination  of  the  optical  character. 


I  ni.  7. — The  appearance  of  a 
biaxial  positive  (+)  crystal, 
showing  one  melatope  in  the 
field,  under  a  gypsum  plate. 


FIGS. 8-9. — Movement  of  the  colors 
upon  inserting  a  quartz  wedge  above 
the  interference  figure  of  a  positive 
(  +  )  biaxial  mineral.  The  lower  nr- 
rows  indicate  the  direction  of  c  in  the 
wedges. 


Fi<i.  10. — A  biaxial  interference  figure 
seen  in  two  positions,  each  field  xhowing 
the  emergence  of  an  optic  a\i-.  The 
arrows  indicate  the  movement  taking  place 
upon  inserting  a  quartz  wedge  alx>ve  the 
section.  The  mineral  is  augite  (+). 


The  optical  character  of  both  uniaxial  (457-62)  and  biaxial  (462-65)  crystals  is  obtained,  as  just 
mentioned,  in  sections  which  show  the  emergence  of  an  optic  axis.  The  determinations  may  be 
made  by  means  of  various  accessories,  the  ones  most  commonly  employed  being  the  gypsum  plate 
and  the  quartz  wedge. 


40  ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 

If  one  considers  a  uniaxial  crystal  as  only  a  special  case  of  a  biaxial  in  which  the  optic  axial  angle  is  0°,  or  con- 
siders the  biaxial  as  a  special  case  of  the  uniaxial  in  which  the  axis  has  been  split  apart,  one  need  only  remember  the 
phenomena  in  two  cases.  Figure  4  shows  a  uniaxial  interference  figure  with  the  emergence  of  the  optic  axis  in  the 
center  of  the  field.  In  a  positive  mineral,  with  the  gypsum  plate  orientated  as  shown  in  the  figure,  the  blue  spots 
appear  in  the  northeast  and  southwest  quadrants.  (If  the  orientation  of  the  slow  ray  in  the  gypsum  plate  is  at  right 
angles  to  that  shown  in  the  figure,  the  phenomenon  is,  of  course,  reversed,  and  yellow  appears  in  the  northeast,  etc., 
quadrants.)  Figure  5  shows  the  phenomenon  appearing  when  a  quartz  wedge  whose  slow  direction  vibrates  across  the 
accessory  is  pushed  across  a  positive  uniaxial  figure.  One  needs  but  remember  that  the  colors  in  the  south- 
east quadrant  move  toward  the  hand.  (With  negative  minerals  or  with  the  orientation  of  the  slow 
ray  in  a  different  direction  in  the  wedge,  the  phenomenon  is  reversed.) 

All  other  cases  can  be  reduced  to  these  two.  Thus  Figures  6  and  7  are  the  same  as  Figure  4,  and  Figures  8,  9,  and 
10  the  same  as  Figure  5. 

Measurement  of  the  optic  axial  angle.— The  methods  for  measuring  the  optic  axial  angle  should 
be  looked  up  elsewhere  (466-502). 

A  QUANTITATIVE  MINERALOGICAL  CLASSIFICATION  OF  IGNEOUS  ROCKS 

The  main  division  lines  in  the  classification  of  igneous  rocks  which  is  generally  accepted  are  the 
result  of  a  gradual  development  through  the  hundred  and  twenty  years  during  which  were  produced 
the  systems  of  Werner  (1787),  von  Leonhard  (1823),  Zirkel  (1866  and  1894),  and  Rosenbusch  (1877, 
1887,  1897,  and  1907-8).  All  of  these  systems  were  qualitative  and  more  or  less  mineralogical,  but 
they  lacked  the  quantitative  element  now  deemed  essential.  As  a  consequence,  rock  terms  have 
been  used  loosely  or  with  different  meanings.  Thus  dolerite,  originally  applied  to  a  coarse  basalt, 
has  been  used  for  any  dark  rock,  and  in  England  is  used  for  rocks  which  we  call  diabase.  The  term 
diabase  in  the  United  States  means  a  dike-rock  with  an  ophitic  texture,  yet  it  was  originally  used  for 
Paleozoic  basalts  and  is  still  so  used  in  various  countries.  Basalt  has  been  applied  to  plagioclase 
rocks  with  augite  and  olivine  and  irrespective  of  the  kind  of  feldspar,  to  labradorite-pyribole  rocks 

with  or  without  olivine,  to  the  darker  labradorite-pyribole  rocks, 
to  post-Tertiary  extrusives  of  gabbroic  magma,  etc. 

The  system  to  be  presented  here1  is  strictly  mineralogical, 
quantitative,  and  modal,  and  is  directly  applicable  to  all  plutonites 
and  to  practically  all  extrusives.  No  attempt  has  been  made 
to  change  the  general  basis  of  classification  of  the  old  system, 
although  the  additional  factor  of  the  ratio  of  the  dark  to  the 
h'sht  constituents  is  used. 

As  an  objection  to  a  quantitative  mineralogical  system,  it  will 
be  said  that  it  is  not  always  possible  to  determine  the  exact  com- 
Foide  position  of  rocks  with  glassy  bases,  or  extrusives  of  the  alkali  series. 

pia  11 Subdivisions  of  the  double      But  the   percentage  of  indeterminable  rocks  is  comparatively 

tetrahedron  into  classes,  representing     small,  and  for  these  there  still  remain,  if  necessary,  chemical 
light  to  dark  rocks.  methods  for  determining  the  composition  of  the  base.     Most 

glassy  rocks  are  leucocratic,  and  a  recalculation  into  the  minerals 
which  would  have  crystallized  had  the  conditions  been  right  is  easy. 

The  basis  of  the  classification  here  proposed  is  a  double  tetrahedon  (Fig.  11),  each  trihedral 
angle  of  which  represents  certain  mineral  constituents.  Since  there  is  no  geometrical  figure  having 

1  This  classification  of  igneous  rocks  has  been  gradually  developed  by  the  writer  since  1909,  and  is  described  in 
considerable  detail  in  the  following  papers,  to  which  the  reader  is  referred: 

"Suggestions  for  a  Quantitative  Mineralogical  Classification  of  Igneous  Rocks,"  Jour.  Geol.,  XXV  (1917),  63-97. 
Figs.  27. 

"A  Quantitative  Mineralogical  Classification  of  Igneous  Rocks,  Revised,"  ibid.,  XXVIII  (1920),  38-60,  159-77, 
210-32.  Figs.  7. 


OF   ROCK-FORHINO    MINERALS   AND    ROCKS  41 

as  many  comers  as  there  are  minerals  in  the  rocks,  it  was  fount!  necessary  to  divide  the  minerals  into 
certain  groups.  Since  quartz  and  the  feldspathoids  never  occur  together,  it  is  possible  to  make  the 
classification  in  five  dimensions  by  usinn  two  tetrahedrons  with  a  common  base. 

The  groups  of  minerals  represented  by  the  corners  of  the  double  tetrahedron  are  (1)  quartz 
(symbol  t^u1);  (2)  jxitash  feldspars,  including  also  anorthoclase,  microperthite,  etc.  .(symbol  Kf), 

(3)  all  planioelases,  (4)  all  fcldspathoids,  (5)  the  mafites,1  including  the  ferromagnesian  constituents, 
the  "ores,"  etc.,  as  given  below. 

As  shown  in  Figure  11,  the  double  tetrahedron  is  unsymmetrically  divided  by  the  traces  of  planes, 
some  parallel  to  the  qiiarfeloid*  face,  others  converging  to  one  of  the  angles.  The  divisions  were  so 
made  to  conform  to  the  rock  names  of  the  older  classifications.  It  is  true  that  new  names  might 
have  l>oen  devised  for  symmetrical  subdivisions,  but  it  was  not  thought  desirable  to  discard  entirely 
the  old  and  well-tried-  lines  of  separation  which  have  very  much  to  recommend  them  besides  the  fact 
that  they  have  been  so  long  in  use.  The  old  classifications  are  unsymmetrical,  for  we  speak  of  a  rock 
as  a  quart /->\enite,  quartz-monzonite,  quartz-diorite,  etc.,  when  it  contains  any  amount  of  quartz. 
With  resjH'ct  to  this  mineral,  therefore,  the  classification  is  based  upon  its  ratio  to  the  sum  of  all  the 
other  constituents,  and  the  lines  of  division  must  be  parallel  to  the  side  of  the  tetrahedron.  The 
same  is  true  also  of  the  feldspathoids.  In  the  divisions  according  to  the  feldspars,  however,  we  find 
for  example  that  a  rock  is  a  quartz-monzonite  whether  the  total 
feldspar  percentage  is  10  or  90.  Here  the  divisions  are  based 
upon  the  ratio  of  the  feldspars  to  each  other,  irrespective  of  what 
their  amount  may  be  in  the  rock.  The  division  lines,  therefore,  Order 
must  converge  toward  the  quartz  and  feldspathoid  corners,  as 
shown  in  Figures  11,  15,  etc. 

Classes. — The  igneous  rocks  may  be  divided  into  various 
classes  according  to  the  percentage  of  dark  constituents  present. 
Four  divisions  are  here  made:  (1)  rocks  with  less  than  5  per  cent 
of  dark  constituents,  (2)  dark  constituents  between  5  and  50 
per  cent,  (3)  dark  constituents  between  50  and  95  per  cent,  and 

(4)  dark  constituents  more  than  95  per  cent.     Since  these  Folds 

division  lines  represent  planes  parallel  to  the  two  quarfeloid  Fio.  12.— Subdivisions  of  the  second- 
planes  (quartz-feldspars  and  feldspars-feldspathoids),  Figure  11,  wy  double  tetrahedron   into  orders, 
,                                             .   .        ,           ,                                              .1  representing  differences  in  the  kind  of 
they  form  similar  double  tnangles  whose  sizes  represent  the  piLrioclase 

amounts  of  light  constituents,  consequently  decrease  with  in- 
crease in  dark  constituents  and  with  approach  to  the  mafite  comer.     For  convenience,  however, 
since  they  are  similar,  they  may  be  represented  by  triangles  of  the  same  size. 

Orders. — Thus  far  the  classification  is  one  of  five  dimensions.  But  this  is  not  enough.  The  kind 
of  plagioclase  in  the  rock  must  be  taken  into  consideration.  Imagine  that  the  lozenge-shaped  quarfe- 
loid plane  consists  of  two  sheets  of  paper  fastened  together  only  along  the  Qu-Kf-Foids  edge.  If  now 
the  loose  corners  at  the  right  of  the  two  sheets  be  separated  a  distance  equal  to  a  side  of  the  original 

1  In  the  figures  following,  the  quartz  corner  is  indicated  by  the  symbol  Qu.  The  letter/  is  used  for  feldspar,  there- 
fore Kf  indicates  the  potash-feldspars — orthoclase,  microcline,  anorthoclase,  microperthite,  etc.  Naf  indicates  albite, 
CaA'of  represents  the  acid  plagioclases,  XaCaf  the  basic  plagioclaaes,  and  Caf  anorthite.  In  CaJVof  and  NaCof,  the 
element  in  excess  is  indicated  by  italics  and  the  symbols  are  to  be  read,  calcium-bearing  soda-feldspar,  and  soda-bearing 
calcium  feldspar.  Folds  is  the  symbol  used  for  the  feldspathoids. 

'  The  term  mafite  is  here  used  for  the  dark  minerals  of  a  rock.  This  term  includes  not  only  the  mafic  (ferromag- 
nesian) minerals  of  C.I.P.W.,  but  certain  iron  minerals  listed  below,  as  well. 

'  Quarfeloids  (Ql'ARU,  FELdspar,  feldspathUlDS)  is  used  as  a  noun  for  minerals  in  the  front  face*  of  the  double 
tetrahedron,  "felsite"  being  unavailable  from  its  use  as  a  rock  name.  " Leucocrates "  cannot  be  used,  since  all  light- 
colored  minerals  are  not  included. 


42 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


triangle,  a  new  double  tetrahedron  will  be  developed,  the  horizontal  line  along  which  it  was  opened 
representing  all  plagioclases,  the  ends  being  formed  by  the  Ab  and  the  An  molecules  (Fig.  12).  The 
same  thing  is  done  in  each  of  the  first  three  classes,  the  fourth  being  differently  divided  as  shown 
below.  The  classification  will  now  be  made  up  of  three  double  tetrahedrons  (and  a  single  tetrahedron 
for  the  fourth  class),  one  for  each  class,  the  corners  being  formed  by  quartz,  potash  feldspar  (including 
microperthite,  anorthoclase,  etc.),  albite,  anorthite,  and  the  feldspathoids.  But  these  tetrahedrons 
may  be  subdivided  into  orders,  depending  upon  the  proportions  of  the  albite  to  the  anorthite  mole- 
cule; consequently  the  divisions  must  be  made  by  planes  all  of  which  cut  the  quartz-potash-feldspar- 
feldspathoid  edge  but  which  separate  across  the  central  plane  of  the  double  tetrahedron  as  shown 
by  the  dotted  lines  in  Figure  12  and  by  Figure  13.  The  edge  Qu-Kf-Foids  remains  common  to  all 
of  the  divisions,  the  plagioclase  corner  simply  having  been  changed.  Now  while  the  triangles  formed 
by  the  intersections  of  these  planes  with  the  tetrahedron  are  not  all  equilateral,  the  relative  position  of 
any  rock  plotted  on  any  intersecting  plane  is  the  same  as  it  would  be  in  the  equilateral  triangle,  since 
the  divisions  are  100  each  way  in  every  possible  triangle.  Equilateral  triangles,  consequently,  may 


Pyroxend 


FIG.    13. — A    section    through    the 
central  plane  of  Figure  12. 


FIG.  14. — Subdivisions  of  the  single 
tetrahedron  of  Class  4  into  orders. 


be  substituted  for  any  triangle.  In  this  manner  the  different  orders  may  be  represented  by  a  series 
of  double  equilateral  triangles  whose  right-hand  corners  vary  with  the  kind  of  feldspar.  Figures  16-19 
show  the  plutonic  rocks  in  Class  2. 

Each  of  the  first  three  classes  is  hereby  divided  into  orders  according  to  the  Ab-An  ratio  in  the 
plagioclase.  The  division  points  are  Abi00An0,  Ab96An6,  Ab6oAn6o,  AbsAngs,  AboAnioo.  There  are 
thus  formed,  for  each  class,  four  double  triangles  in  each  of  which  three  angles  represent  (1)  quartz 
(Qu),  (2)  all  feldspars  except  plagioclase  (Kf),  and  (3)  the  feldspathoids  (Foids).  The  remaining 
angle  (Flag)  represents  albite  (Naf)  in  Order  1,  oligoclase  to  andesine  (CaNat)  in  Order  2,  labradorite 
to  bytownite  (NaCaf)  in  Order  3,  and  anorthite  (Caf)  in  Order  4,  making  the  divisions  conform  to  the 
present  lines  of  separation  between  the  alkali  rocks,  the  acid  plagioclase  (dioritic)  rocks,  the  basic  plagio- 
clase (gabbroic)  rocks,  and  the  anorthite  rocks.  Figures  16-19  represent  the  triangles  of  Class  2.  There 
are  similar  sets  for  Classes  1  and  3.  Zonal  feldspars  may  be  determined  by  considering  the  approximate 
amounts  of  each  kind  and  obtaining  the  average  Ab-An  value.  This  will  be  necessary  in  but  few 
cases,  for  ordinarily  a  simple  inspection  will  show  whether  the  average  runs  across  the  AbsoAriso  line. 
Of  course  if  the  nucleus  as  well  as  the  rim  falls  entirely  between  the  0  and  5,  5  and  50,  50  and  95,  or 
95  and  100  lines,  there  is  no  need  for  computation. 

Class  4 :  Owing  to  the  practical  absence  of  light  constituents  in  Class  4,  the  subdivisions  into  orders 
must  be  made  on  a  different  basis  from  those  of  the  first  three.  They  are  here  made  by  dividing  the 
tetrahedron  by  planes  parallel  to  the  left-hand  face,  forming  four  orders,  depending  upon  the  amount  of 
ores  present  (Fig.  14).  The  division  points  for  these  planes,  as  in  the  other  classes,  are  0-5-50-95-100. 

There  are  now  six  dimensions  in  the  classification,  and  since  each  pigeonhole  will  represent  not 
only  a  plutonic  rock  but  also  a  hypabyssal  and  an  extrusive,  we  may  say  we  have  a  classification  in 


OF  RnrK-FoRMiN-o  MIVKRALS  AVD 


43 


seven  dimensions,  yet    every    rock   is  shown   by   a  single   point  on   a  drawing 
in    a   single    plane.     The  more  detailed  description  which  follows  may  make  this  clearer. 

Families. — The  quarfeloid  face  of  the  double  tetrahedron  will  appear  a-  >hown  in  Figures  Hi  I'.i. 
The  families  in  the  first  three  classes  are  to  be  mimlx-rod  as  shown  in  Figure  15.  The  object  of  begin- 
ning with  0  is  to  make  the  positions  easier  to  remember,  since  they  run  in  groups  of  four.  The  division 
points  are  at  0-5-50-95-100  on  lx>th  the  feldspar  base  line  and  in  the  vertical  direction,  except  for  tin- 
few  intermediate  mon/onitic  families  to  lx»  mentioned  in  a  moment.  Families  0,  1,  5,  9,  13,  17,  21, 
and  25  occur  but  once  in  each  class,  since  the  amount  of  plagioclase  in  each,  whether  albite,  acid 
plagioclasc,  basic  plagioclaso,  or  anorthite,  is  too  small  to  make  an  essential  difference  in  the  rock. 
They  form  the  hinge  alwut  which  the  order  tetrahedron  (Fig.  12)  was  opened,  and  are  the  same  in  all 
orders.  For  convenience  these  "hinge  families"  are  classed  with  Order  1.  This  is  shown  in  Figures 
16-10  where  these  families  arc  omitted  and  are  represented  by 
dot  tod  lines. 

In  the  original  article  describing  this  classification  there  were 
thirty-two  families,  an  intermediate  family  having  been  inserted 
along  the  AbsoAnjo  line,  namely,  the  family  of  the  monzonites. 
Since  in  general  descriptions  these  monzonitic  rocks  are  unneces- 
sary, they  are  omitted  from  all  orders.  For  special  rock  de- 
scriptions the  additional  monzonitic  families,  adamellite  and 
mon/onite,  are  used  as  shown  in  Figure  15.  Certainly  the 
intermediate  families  are  not  necessary  between  Families  2  and  3, 
22  and  23,  and  doubtfully  between  14  and  15,  and  18  and  19. 
Whether  they  are  used  or  not  need  cause  no  confusion,  the  rock 
names1  as  indicated  show  what  divisions  have  been  made.  That 
these  are  smaller  families  is  shown  even  in  the  numbers  by  the 
marks  (')  and  (")  after  them.  They  may  thus  be  used  or  not 
as  desired. 

Class  4 :  The  single  tetrahedrons  of  the  four  orders  of  Class  4 
are  subdivided  on  the  basis  of  the  dark  minerals  present  since  the 
light-colored  constituents  are  practically  wanting.  In  Orders  1, 
2,  and  3,  of  Class  4,  the  corners  represent  respectively  olivine, 
biotitc  and  (or)  amphibole,  and  pyroxene  (Figs.  14  and  20).  In  Order  4,  if  thought  desirable,  they 
may  be  taken  to  represent  the  various  ores:  the  writer,  however,  groups  these  in  one  family,  for, 
considered  as  rocks,  they  are  unimportant.  The  various  hematite,  magnetite,  ilmenite,  etc.,  ores  may 
be  made  subfamilies. 

THE  MINERAL  GROUPS 

The  constituents  of  the  rock  are  divided  into  three  primary  groups: 


Fio.    15. — Family    numbers    in 
Classes  1-3. 


QUARFELOID8 

(Qu)  Quartz 

(Kf)  Orthoclase,  microcline,  microperthitc,  anorthoclase,  etc. 

(Flag)  The  whole  isomorpHous  Ab-An  series  of  plagioclases 

(Foids)  The  feldspathoids  (nephelite,  leucite,  sodalite,  haflynite,  noselite,  melilite,  primary  analcite,  etc.) 

1  Granodiorite  is  given  the  original  significance  intended  by  Lindgren,  being  applied  to  a  rock  intermediate  between 
granite  and  diorite.  Syenodiorite,  as  originally  used  by  the  writer,  is  the  quartz-free  equivalent.  In  the  limited  MOM 
rocks  between  adamellites  and  quarti  diorites  are  called  monzotonalites,  and  lietween  monzonites  and  diorite*  mon- 
zodiorites.  On  the  granite  side  the  limited  granite,  between  orthogranite  and  adamcllitc  is  monzogranite,  and  between 
monzonite  and  orthoeyenite,  monzosyenite.  The  name  indicates  at  once  its  intermediate  position. 


44 


ESSENTIALS  FOB  THE  MICROSCOPICAL  DETERMINATION 


Ortbotyenii 

Kf( 


'  (  Atbite-»d»mellite  ) 

| 

Ennyenitc  i— (  Ubilc-monwinil*  ) W'Albitt  mnninrtiori 

aittynile  _  |  \   X 


PubUctte    \ 


Nat 


Fig.  16. 


Folds 


/    N>;.hf1itf-bc»rlnl 


Fig.  17     v 


,  Mnmoptabbro  ) X-  G.bbro,  Nmiw 

'1"""bl"°       : 4-A  NaCaf 


Fig.  19      x- 


Foids 


Foidi 


OK  Ho.  K-I'uiiMiMi  MINERALS  AND  ROCKS  45 

UAFITB8 

Dark  mica.-  (biotitr,  phlogopitc,  zimiwaMite. 

Amphibolr- 

Pyroxenes  (incluilini:  uralitizeil  pyroxene) 

Olivine 

Iron  "ores"  (magnetite,  ilmenite,  elirumite,  pyrite,  lii-matite,  etc.) 

Cassitcritc 

{darnel 
Primary  epiilote 
Allanite.  zircon,  rutile,  primary  titanitc,  spinel,  and  other  dark  minor  constituents 

AUXILIARY  CONSTITUENTS 

Tlif  auxiliary  constituents  are  seldom  of  importance. 

Topaz  Corundum  Primary  scapolite  Lcpidolite 

Tourmaline  Fluorite  Primary  calcite  Apatite, 

Conlierite  Andalusite  Muscovite  etc. 

Most  of  the  auxiliary  constituents  arc  light  in  color;  they  are,  consequently,  computed  among  the 
IriK-ocrates. 

SECONDARY  CONSTITUENTS 

inlary  constituents  arc  to  be  calculated  as  the  originals  from  which  they  came.  Thus  ore 
replacement-  of  the  mufites  are  computed  as  mafitcs,  kaolin  as  feldspar,  etc.,  chlorite  as  a  biopyribole, 
analcitc  as  fcldspathoid,  pscudoleucite  as  leucite,  etc. 

GLASS 

Glass  must  be  computed  from  an  analysis.  One  can  usually  surmise  its  composition  from  the 
character  of  the  phenocrysts  and  the  appearance  of  the  rock  as  a  whole.  When  undetermined,  the 
rock  must  be  given  a  tentative  name,  such  as  hyaline-rhyolite,  etc.  Glassy  rocks  are  rare. 


RULES  FOR  COMPUTING  ROCKS  FROM  THEIR  MODES 

1.  The  sum  of  the  minerals  in  the  mode  should  be  100±0.5.     If  greater  or  less,  it  should  be  recal- 
culated to  100.    The  sum  of  the  leucocratcs  (quarfeloids  plus  auxiliary  minerals)  determines  the 

r!a-- . 

Class  1.  Leucocratcs  form  more  than  95  per  cent  of  the  total  components 
Class  2.  Leucocrates  between  95  (inclusive)  and  50  per  cent 
Class  3.  Leucocrates  between  50  (inclusive)  and  5  per  cent 
Class  4.  Leucocratcs  between  5  (inclusive)  and  0  per  cent 

2.  Determine  the  orders  in  Classes  1,  2,  and  3  directly  from  the  Ab-An  ratio  in  the  plagioclase. 

Order  1.  Ab,wAn0  to  Ab»An, 
Order  2.  Ab.iAnt  (inclusive)  to  AhioAnM 
Order  3.  AbwAnM  (inclusive)  to  AbtAnu 
Order  4.  AbtAn*  (inclusive)  to  AboAnm 

In  Class  4  the  orders  are  determined  by  the  percentage  of  "ores."  Reduce  the  sum  of  biotite, 
olivine,  pyribole,  and  "ores"  (including  cassiteritc,  chromite,  etc.)  to  100,  dropping  the  minor  mafites, 
apatite,  garnet,  perofskite,  any  small  amount  of  quarfeloids,  etc.  The  percentage  of  "ores"  determines 

the  order. 

Order  1.  0  to  5  per  cent  "ores" 
Order  2.  5  (inclusive)  to  50  per  cent  "ores" 
Order  3.  50  (inclusive)  to  95  per  cent  "ores" 
Order  4.  95  (inclusive)  to  100  per  cent  "ores" 


46  ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 

3.  Determine  the  family.     In  Classes  1,  2,  and  3,  first  recalculate  the  quarfeloids  to  100.     The 
amount  of  quartz  (or  feldspathoid)  thus  determined  immediately  locates  a  row  of  horizontal  pigeon- 

holes,  in  one  of  which  the  rock  belongs.  Recalculate  Kf  plus 
plagioclase  to  100  and  determine  the  proper  point  on  the  Kf-Plag 
base  line.  This  determines  the  vertical  series  of  pigeonholes, 
and  its  intersection  with  the  horizontal  series  gives  the  proper 
position  for  the  family.  Still  simpler  is  the  location  of  the 
family  graphically.  This  is  discussed  below. 

In  Class  4,  Orders  1,  2,  and  3,  recalculate  the  olivine,  pyrox- 
ene, and  biotite  plus  amphibole  to  100  (Fig.  20)  and  find  the 
Biottta££l      10        I         11     ^5&         position  by  taking  the  relation  of  olivine  to  the  sum  of  the  bio- 

AopniDOl«  pyroxene 

FIG.  20.-Family  numbers  in  Class  4       tite  and  Pyriboles  for  the  horizontal  line,  and  of  biotite  plus 

amphibole  to  pyroxene  for  the  intersecting  line.     Graphically  the 
method  is  the  same  as  for  the  other  classes. 

4.  Subfamilies. — In  all  classes,  subfamilies  are  based  on  0-5-50-95-100  division  points  after  the 
manner  shown  in  Figure  21.     Thus  we  may  have  biotite-granite,  hornblende-bearing  biotite-granite, 
biotite-bearing  hornblende-granite,  and  hornblende-granite. 

Biotite-  Hornblende  bearing  biotite-  Biotite  bearing  hornblende-  Hornblende 
granite granlta ,         granite ,  granite  | 

SO 

FIG.  21. — Method  of  naming  subfamilies 


06  50  9B  100 


A    FEW    POINTS   TO   BE    OBSERVED 

A  rock  whose  percentage  value  falls  exactly  on  a  family  line  should  be  given  the  name  of  the 
family  in  the  pigeonhole  toward  the  opposite  apex  of  the  triangle,  except  as  indicated  below.  Thus  a 
syenite  with  5  per  cent  quartz  is  called  a  granite.  On  the  50-50  line  of  quartz  they  are  moved  upward, 
and  on  the  50-50  line  of  foids  downward,  toward  the  apices;  that  is,  they  are  placed  in  Families  1-4 
or  21-24.  On  the  50-50  line  between  Kf  and  Flag  they  are  included  in  the  Flag  side.  Rocks  falling 
on  the  line  separating  the  two  triangles,  namely,  on  the  feldspar  base  line,  usually  should  be  classed 
on  the  quartz  side,  that  is,  on  the  normal  side,  but  if  the  rock  has  affinities  with  alkalic  rocks,  it  should 
be  placed  on  the  Foid  side. 

For  classificatory  purposes,  it  is  seldom  necessary  to  make  exact  determinations  of  the  mineral 
percentages.  Unless  the  proportions  are  such  that  the  rock  falls  near  a  division  line,  a  simple  inspec- 
tion will  answer. 

EXAMPLE 
A  granodiorite  having  the  composition : 

Quartz..  .   14.0  =   18.7 

Orthoclase 15.0  =  20.0=  24.6 

Oligoclase  (Ab90An10) 46.0  =  61 .3=  75.4 

Total  quarfeloids 75.0    100.0    100.0 

Biotite 12.0 

Hornblende 12.0 

Magnetite 0.6 

Titanite..,                                      0.4 


Total  mantes 25.0 


100.0 

Percentage  quarfeloids  =  75.    The  rock  belongs  to  Class  2. 
Ab90Anio  falls  between  95  and  50.     The  order,  therefore,  is  2. 


OF   ROCK-FORMING    MINERALS   AND    ROCKS 


47 


The  family,  with  18.7  quartz  (light  constituents  reduced  to  100),  falls  in  the  row  of  families  5  to  8 
The  nilin  orthodase  t<>  oligoclase  is  24.6  to  75.4,  consequently  the  family  is  7,  granodiorite.  If  the 
mon/onitic  families  are  included,  it  is  7',  mmizotonalite.  The  rock  number  is  227  (or  227'),  to  be 
read  two,  two,  seven  (or  two,  two,  seven  prime). 

( Jraphieally1  the  rock  may  l>e  shown  by  a  point  and  a  line  as  indicated  in  Figure  22.  Draw  three 
lines,  parallel  to  the  sides  of  the  triangle,  through  the  points  representing  the  amounts  of  the  corner 
minerals  in  the  rock.  In  this  case  a  horizontal  line  through  14  for  quartz,  a  line  sloping  northeast  and 
M nit h west  through  40  for  oligoclase,  and  a  line  sloping  northwest  and  southeast  through  15  for  ortho- 
clase.  Lay  the  side  of  a  straight-edge  on  the  apices  of  the  two  triangles  and  draw  a  short  line  through 
the  small  triangle  from  its  apex  to  its  base.  Connect  either  of  the  lower  corners  of  the  large  triangle 
with  the  >imilar  corner  of  the  small  one,  and  indicate  its  intersection  with  the  line  first  drawn  by  a 
dot.  /•'.  The  line  M  and  the  spot  /•'  represent  the  rock.  The  point  /•'  is  the  same  as  that  which  was 
obtained  by  computation  above.  It  gives  the  relative  proportions  of  the  light  constituents,  quartz 
i  nil' i.  orthoclasc  (kF),  and  plagioclase  (IF),  in  the  rock  by  its  distances  from  the  sides  of  the  triangle 
opposite  these  names  at  the 

corners.     The  actual  percent-  Quartz 

ages  of  the  minerals  in  the  rock 
are  also  represented:  be  or  pk 
for  orthoclase,  ab  or  to  for 
plagioclase,  '<</  for  the  dark 
constituents,  dm  for  quartz. 

In  a  similar  manner  the 
composition  of  the  rock  at  L 
may  be  read  directly  from  the 
diagram,  giving  the  values: 

Quartz 23  =  27.7 

Orthoclase 45=  54.2 

Andesine 15=   18.1 


Total  quarfeloids   83     100.0 
Dark  constituents  17 

lob 


Orth. 


S  m 

Fio.  22. — Method  of  graphical  plotting  of  rocks 


Flag. 


Class  2,  Order  2,  Family  6' 
(normal  granite  =  6,  or  monzo- 

granite=6'). 

The  rock  at  S  is  seen  to  consist  of  orthoclase  27  per  cent,  plagioclase  42  per  cent,  quartz  0  per 
cent,  and  dark  minerals  31  per  cent.  The  relative  ratios  of  the  light  constituents  are:  orthoclase 
39.1  per  cent,  plagioclase  60.9  per  cent,  and  quartz  0  per  cent.  The  rock  at  T  consists  of  orthoclase 
0  per  cent,  plagioclase  66  per  cent,  quartz  24  per  cent,  dark  constituents  10  per  cent.  Relative  ratios 
of  the  light  minerals,  plagioclase  73.5  per  cent,  quartz  26.5  per  cent,  and  orthoclase  0  per  cent. 

The  actual  plotting  takes  much  less  time  than  the  telling.  In  fact  the  small  triangle  is  never 
drawn.  The  upper  end  of  the  line  is  located  at  the  dividing  point  between  orthoclase  and  plagioclase 
on  the  horizontal  line  whose  length  is  equal  to  the  sum  of  the  feldspars,  and  through  this  point  the 
inclined  line  is  drawn  to  the  percentage  position  of  quartz.  One  of  the  lower  comers  of  the  small 
triangle  is  located,  without  drawing  it,  and  the  intersection  of  the  inclined  line  with  the  ferromagnesian 
mineral  line  is  used  to  determine  the  locus  of  the  rock. 


'See  the  writer's  "On  the  Representation  of  Igneous  Hocka  in  Triangular  Diagrams,"  Jour.  Oeol.,  XXX  (1922), 
pp.  167-69. 


48  ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 

NAMES  PROPOSED  FOR  VARIOUS  FAMILIES 

On  the  basis  of  the  foregoing  subdivisions,  many  modal  analyses  have  been  plotted  and  names1 
have  been  given  to  many  of  the  families,  most  of  them  derived  from  plutonic  rocks  falling  at  the  center 
points.  In  some  cases,  as  in  the  quartz-rich  types,  family  names  were  taken  from  differentiation 
rocks.  In  the  tabulation  following,  there  are  many  blank  pigeonholes,  owing  to  lack  of  good  modal 
descriptions.  There  are  undoubtedly  many  rocks  in  most  of  the  families  here  left  blank,  especially 
in  Classes  2  and  3,  but  the  majority  of  published  rock  descriptions  lack  mineral  percentages,  making 
them  unavailable  for  classification.  Blank  spaces  in  the  tables  do  not  necessarily  mean  that  rocks 
are  wanting  in  these  pigeonholes,  but  may  indicate  that  none  falls  near  the  center  point,  although, 
on  the  other  hand,  a  solitary  rare  rock  may,  in  some  cases,  give  its  name  to  the  family,  even  though 
it  is  not  at  the  center. 

A  certain  system  is  used  for  the  prefixes.  The  terms  "granite,"  "syenite,"  "diorite,"  etc.,  are 
defined,  and  the  addition  of  a  prefix  to  any  one  indicates  a  definite  modification.  Where  no  specific 
names  are  available,  "leuco-"  is  used  to  indicate  rocks  of  Class  1,  "meso-"  those  of  Class  2,  and 
"mela-"  those  of  Class  3.  In  most  cases  the  prefix  "meso-"  is  unnecessary,  since  normal  rocks 
belong  to  Class  2,  and  these  are  written  without  the  prefix,  the  class  being  understood.  Thus  there 
are  leuco-granites,  granites,  and  mela-granites,  respectively,  in  Classes  1,  2,  and  3. 

Analogous  rocks  in  the  four  orders  of  each  class  similarly  have  distinctive  prefixes  where  no 
other  names  are  available.  The  rocks  of  Order  1  have  albite  as  their  plagioclase;  therefore  an  albite- 
monzonite  is  a  monzonite  whose  plagioclase  is  albite,  and  in  Order  4  an  anorthite-monzonite  is  one 
containing  orthoclase  and  anorthite.  An  albite-diorite  means  a  rock  all  of  whose  plagioclase  is  albite, 
since  a  diorite  contains  only  plagioclase;  an  albite-granite,  on  the  other  hand,  means  a  granite  con- 
taining some  albite  in  addition  to  orthoclase,  since  granite  itself  is  defined  as  a  rock  consisting  of 
quartz,  a  biopyribole,  orthoclase,  and  less  plagioclase.  That  is  to  say,  the  term  "granite"  in  itself 
conveys  the  idea  of  an  orthoclase  rock  with  some  plagioclase,  the  latter  indicated,  except  in  normal 
rocks,  by  the  prefix.  The  plagioclase  in  Order  2  is  oligoclase  to  andesine,  and  that  of  Order  3  labra- 
dorite  to  bytownite.  Acid  and  basic  cannot  be  used  as  prefixes  for  these  orders,  since  albite  and 
anorthite,  the  end  members  of  the  acid  and  basic  plagioclases,  are  set  apart  as  Orders  1  and  2. 
Lime-soda  and  soda-lime  are  so  much  alike  that  one  must  always  stop  to  think  which  is  meant. 
The  prefixes  "sodi-"  and  "calci-"  are  here  suggested.  As  in  the  names  of  normal  classes,  here  also 
normal  rocks  drop  the  prefix;  "sodi-,"  therefore,  is  seldom  necessary.  To  the  rocks  of  the  hinge 
families,  namely,  those  which  contain  no  plagioclase,  "ortho-"  is  prefixed;  the  feldspar  present  is 
orthoclase,  microcline,  microperthite,  or  anorthoclase. 

In  the  classification  as  it  now  stands  the  term  granite  is  applied  to  all  those  rocks  called  granite 
before  the  introduction  of  the  term  monzonite,  that  is,  rocks  with  more  orthoclase  than  plagioclase 
(Family  6).  Granodiorite,  likewise,  is  used  in  its  original  sense,  that  is  for  a  rock  between  granite 
and  diorite  (Family  7).  If  use  is  made  of  the  monzonitic  terms,  the  limited  family  of  granite,  between 
quartz-monzonite  and  orthogranite,  is  called  monzogranite  (6'),  and  the  limited  granodiorite  between 
quartz-monzonite  and  tonalite,  monzotonalite  (?')•  Likewise  the  terms  syenite  (10)  and  syenodiorite 
(11),  when  limited,  become  monzosyenite  (10'),  and  monzodiorite  (!!')•  Quartz-monzonite  or 
adamellite  therefore  takes  a  part  of  6  and  7  as  6"-7",  and  monzonite  10"-11"  (Figs.  16-19). 


1  For  a  list  of  these  names  with  histories  of  the  terms,  see  the  paper  above  quoted,  Jour.  Geol.,  XXVIII  (1920), 
53  ff.  Family  names  for  the  monzonite  group  there  given  are  in  general  to  be  omitted.  The  lists  in  the  succeeding 
pages  are  to  be  followed. 


OF    I!"'  K-FOBimra     MlSK HALS   AND    HOCKS 


49 


TABLE  I 

Quarfi'loi.l"  .  100       .  95 

-^-- 


Unl.T  I 
AbinAn<  10  AbuAn 

Order  2 
Abu  Am  to  AbuAnu 

Orders 
AbwAmt  to  AtxAnM 

Order  4 
AbtAiiM  to  AbtAniK 

0  Silfxitr 

(-110) 

(-110) 

(-110) 

1   (  >rtliiit;ir:uitiilito 

(-111) 

(-111) 

(-111) 

•_•  T:tr:m1iilitc 

i  liMiiitf-Kreisen 

3 

Grnnodiorite-greisen 

4 

Tonal  itc-greiiieD 

.-,   llrtli..:d:i.-kite 

(-115) 

(-115) 

(-115) 

6  Ala-kit,' 

Leurogrenite 

7  I.iMin>-:dl>iti'-itr!iin>diiirite 

Lcucogranodiorite 

Leucogranogabbro 

S    1.,  11,  ,,-:ill,itf-ton;llite 

Leucotonalitc- 

Quarti-anorthositc 

9  Orthosite 

(-119) 

(-119) 

(-119) 

in  I..'iiri>-iill)ili-i.yi-iiitc 

IxMioosycnite 

1  1   I.euco-albite-sycnodiorit<' 

Leucoeyenodiorite 

Iveucoeyenogabbro 

Lcuco-anorthitc-«ycnogabbro 

12  Albitite 

Ix!uc«xliorite 

Anorthositc 

Anorthitite 

13 

(-1113) 

(-1113) 

(-1113) 

14 

15 

16 

Dungannonite 

17 

(-1117) 

(-1117) 

(-1117) 

18 

19.LeucolitchfieIdite 

•JO  Ix'iicoinariupolite 

21 

(-1121) 

(-1121) 

(-1121) 

22 

23 

24 

Craigmontite 

25 

(-1125) 

(-1125) 

(-1125) 

: 


With  the  additional  families: 


6' 

Lcucomonzogranite 

6"-7"  Lcuco-albitc-adamellite 

Ixmcoadamellite 

7' 

I/eucomonzotonalite 

10' 

I^uconionzosyeniti- 

10"-11"  Leuco-albite-monzonite 

Leucomonzonite 

11' 

Leucomonzodiorite 

50 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


CLASS  2. 


TABLE  II 

Quarfeloids 


Mafites 


between  -=-  and  _-. 
5          50 


Order  1 
AbiooAno  to  AbjsAm 

Order  2 
AbisAm  to  AbsoAnio 

Order  3 
AbwAnso  to  AbsAnss 

Order  4 

AbsAn»i  to  AboAniw 

0  Mesosilexite 

(  =  210) 

(  =  210) 

(  =  210) 

1  Moyite 

(=211) 

(  =  211) 

(=211) 

2 

Quartz-granite 

3 

Quartz-granodiorite 

4  Rockallite 

Quartz-tonalite 

5  Orthogranite 

(  =  215) 

(  =  215) 

(=215) 

6  Albite-granite 

Granite 

Calcigranite 

Anorthite-granite 

7  Albite-granodiorite 

Granodiorite 

Granogabbro 

Anorthite-granogabbro 

8  Albite-tonalite 

Tonalite 

Quartz-gabbro 

Quartz-anorthite-gabbro 

9  Orthosyenite 

(  =  219) 

(  =  219) 

(  =  219) 

10  Albite-syenite 

Syenite 

Calcisyenite 

Anorthite-syenite 

11  Albite-syenodiorite 

Syenodiorite 

Syenogabbro 

Anorthite-syenogabbro 

12  Albite-diorite 

Diorite 

Gabbro,  Norite 

Anorthite-gabbro 

13  Pulaskite 

(=2113) 

(  =  2113) 

(  =  2113) 

14 

Nephelite-bearing 
syenite 

15 

Nephelite-bearing 
syenodiorite 

16 

Nephelite-bearin  g 
diorite 

17  Ortho-nephelite-syenite 

(=2117) 

(  =  2117) 

(  =  2117) 

18  Albite-nephelite-syenite 

Nephelite-syenite 

19  Litchfieldite 

Nephelite-syenodiorite 

Nephelite-syenogabbro 

20  Mariupolite 

Nephelite-diorite 

Nephelite-gabbro 

21  Naujaite 

(=2121) 

(  =  2121) 

(.  =  2121) 

22  Beloeilite 

Heronite 

23 

24  Toryhillite 

Lugarite 

25  Urtite,  Fergusite,  Uncompahgrite 

(  =  2125) 

(  =  2125) 

(  =  2125) 

With  the  additional  families: 


6' 

Monzogranite 

6"-7"  Albite-adamellite 

Adamellite 

Calciadamellite 

Anorthite-adamelh'te 

7' 

Monzotonalite 

10' 

Monzosyenite 

10"-11"  Albite-monzonite 

Monzonite 

Calcimonzonite 

Anorthite-monzonite 

11'  Albite-monzodiorite 

Monzodiorite 

Monzogabbro 

Anorthite-monzogabbro 

OF  RncK-FoRMi\<:  MIVV.RALH  AND  ROCKS 


51 


TABLE  III 


Quarfcloi.ls 
CLAM3'       Mafite. 


50 


OrdiT  1 
AbioAni  to  AlHi.Mu 

Order  2 
AhitAm  to  AbwAnH 

Orders 
At)-  An-  to  AbiAnH 

Order  4 
Ab»An»  to  AthAm. 

0 

(-310) 

(-310) 

(-310) 

1 

(-311) 

(-311) 

(-311) 

2 

3 

4 

.".  Mi'laorthogranit*- 

(-315) 

(-315) 

(-315) 

i;  Mi'la-albite-graaito 

Mrlagranite 

Melacalcigranite 

7  Mi-la-albiU>-Kranodiorite 

M  I'laftranodiorite 

M  elagranogabbro 

8  Mela-albite-tonalite 

Melatonalite 

M  eU-quarti-gabbro 

9  Melaorthosyenite 

(-319) 

(-319) 

(-319) 

10  Mrla-albite-«yenite 

Melasyenite 

1  1    M  rln-albite-«ycnodiorite 

MrLaayenodiorite 

MelaHypnogabhro 

llirol<-ttait<- 

IJ  Mi-lii-nll>it(Mliorite 

Moladiorite 

Melagabbro 

Yamaakitc 

13  OrthoKhiuikinitf 

(-3113) 

(-3113) 

(-3113) 

14  Shonkinite 

OligoclaBe-(andeeine-) 
shonkinite 

Labradorite-  (by  town- 
ite-)  shonkinite 

15 

16 

17  Nephelite-ehonkinite 

(-3117) 

(-3117) 

(-3117) 

18 

19  Melalitchfieldite 

Mela-nephelite- 
syenogabbro 

20  Melamariupolite 

Theralite 

21 

(-3121) 

(-3121) 

(-3121) 

22 

23 

24 

25  Bekinkinite,  Missouritf.Farrisite 

(-3125) 

(-3125) 

(-3125) 

With  the  additional  families: 


8' 

Melamonzogranite 

6"-7"  MeU-albite-adamellite 

Mela-ad  amellite 

Melacalciadamellite 

7' 

Melamonzotonalite 

10* 

Melamonxoeyenite 

10"-11"  Mekpalbite-monsonite 

Melamonzonite 

Melacalcimonxonite 

11' 

Melamonrodiorite 

52 


ESSENTIALS  FOR  THE  MICROSCOPICAL  DETERMINATION 


TABLE  IV 

Quarfeloids ,  5        .0 

CLASS  4.        --  „ between  ==  and  ,,.7: 

Mafites  95         100 


Order  I 
"Ores"  less  than  5  per  cent 

Order  2 

"Ores"  between  5  and 
50  per  cent 

Order  3 
"Ores"  between  50  and 
95  per  cent 

Order  4 
"Ores"  more  than  95  per  cent 

0  Dunite 

Chromite-dunite, 
Magnetite-dunite 

Olivinc-chromitite, 
Olivine-magnetitite 

Chromitite,  Magnetitite 

1  Hornblende-dunite 

2  Pyroxene-bearing-hornblende- 
dunite 

3  Hornblende-bearing-pyroxene- 
dunite 

4  Pyroxene-dunite 

5  Mica-peridotite,  Amphibole- 
peridotite,  Scyelite,  Cortlandite 

6  Olivinite  (  ?) 

Valbellite  (  ?) 

7  Wehrlite 

Koswite 

8  Lherzolite,  Diallage-peridotite, 
Saxonite 

Harzbergite 

9  Amphibolites,  Hornblendites, 
Biotite-pyroxenites 

10 

Olivine-bahiaite 

11 

Bahiaite,  Cromaltite 

12  Diallagite,  Bronzitite, 
Hypersthenite,  Websterite 

Ilmenite-enstatitite, 
Magnctite-pyroxenite 

MINERAL  INDEX 

in  Ixil.l-fare  ty|n-  refer  to  the  group  deacriptiomi) 


Arinit.-  •_>'.».  36 
Actinolitc  I'.',  is.  .'7.  35 
te  -".'.  35 

Argirile-iiuijile  '27.  36 
Albilc   ID.  30 

.Miami.-    See  Drtliilc)  27 


Aliinitc  ."i 

Aiiiphil«>lc-.  iiinnnclinie  36 

AniphilHiie*.  iirtlinrh  .....  Me  36 

Analcitc  :i 

Anala-e  7 

Andaliisite  in.  .'."> 

AndeMiic  '.i 

Anhydrite  i:i 

Anorthita  II.  30 

Aniirthorla-e  '.i.  30,  31 

AnthophyOite  11,  is,  2t>,  36 

Antiisoriic  Id.  17.  jr. 

Anti|M-rthite  '.I 

Apatite  .').    l.V  Jl 


A  rf  \vil.-omtc  27.  36 
Actrophyllil 

mte  II.  is.  27.  36 

Sec  Gthnite) 


Harkevikitc  27.  36 

tic  ImniMciide  27,  36 
Heckelite  :< 
Biotil 

Bronzite  9,  17,  2T>,  36 
Hr.M.kitc  13,  19,  28 
BruciteO 
Bytownite  31 

Calcite  7,  15,  23 

Cancrinite  6 

Cassiterite  6,  15 

CcUi:in  30 

(  'li.'ilccilmiy  10 

Chiiuttnlitc  (Sec  Andalusite) 

Chlorite  (See  Pennine  and  Clinochlore) 

Chronutc  1 

Chrysolite  (See  Olivinc) 

ChryHotile  (See  Serpentine) 

Clin'ochlorelT,  26 

Clinozoisitc  9,  17 

Common  hornblende  26,  36 

Cordicrite  9,  17,  25 

Corundum  6,  21 

Cyanite  (See  Disthene  )10,  17,  25 

Datolite  13 

Desminc  (See  StUbite) 

DiallaRp  12,  18,  27,  36 

Dii'hroite  (See  Cordieritc) 

Diopmdc  12,  18,  27,  36 

Dipyr  6 

Disthene  10,  17,25 

Dolomite  7,  15,  23 

Dumortierite  25 

Enstatite  9,  36 

Kpiilote,  Green  (See  Pbtacitc)  12,  28 

Epixtilbite  10 

Kneolitc  5,  15 

Eudialyte  5,  15 


ite  L3.  I'1 

Kihni  Iliiuanitc) 

Muorile  J 
•.•rite  12 


Garnet*  (See  the  different  varieties)  3 
Qutaldite  36 
Gedrite  18,  27,  36 
Glass  3 

Glaucophane  '2(>,  35 
<  Iraphitc  1 
' 


Grime-rite  29,  36 
Gypsum  10 

HaOyno  (Sec  HaOynito) 
Haii^iito  2 
IfedenlMTKito  IS,  36 
lleiiiiititt-  1.  •_'.'! 
llereynite  :i 
Heulandite  9 

Hnrnlilcnile,  baHaltic  27,  35 
llnnihlendo,  common  26,  36 
Hydroncphelite  5 
Hypcrsthene  11,  26,  35 

Idocrasc  (Sec  Vesuvianitc) 

Ilnienite  1 

lolitc  (Sec  Cordierite) 

Jadcite  36 

Kaolin  9,  17 
Kaolinite  (Sec  Kaolin) 
Katophoritc  36 

Labradorite  9 
Laumontite  11 
Lcpidolite  13 

I  .en,  -iii-  2,  5 
Leucoxenc  13,  19,  28 

Magnesite  7,  15,  23 

Magnetite  1 
Mcinnitc  0 

Melanite  3 

Melilite  5,  15,  21 
Menaccanit^  (Spr  Ilmcnitr) 
Mica  (Sop  (lifTerent  varieties) 
Mieroeline  '.I,  30,  31 
Micnilinc  microperthitc  9 
Microperthite  9 
Mizzonite  (i 
Mi.na/itc  13,  19 
Muscovite  12,  18 

Ncphclinc  (Sec  Ncphclite) 
Xephelitc  5 
Noeean  (See  Noeelite) 
Noselitc  2 

Octahedrito  (See  Anatase) 
(  Hinorlase  9 
(  )liKi>clase-albitc  v 
Olivinc  12,  19,  28 
Opal  3 


Orthitc  .'7 
OrthoclaM'  <l,  30 
Ottrehtc  10,  26 

ParagoniU-  13,  18.  28 

I'lirgamU-  (Giwn  honililemlc)  26,  38 

I'.M-tolite  12,  35 

Pennine  2.~> 

I'erii'lasc  3 

IVrofskite  3 

Phlogopitc  7,  13,  18,  2!l 

Picotitc  1,  H 

Pfodmoatra  -".' 

PisUriti    1'J.  L's 
I  Mam.  H-la-c  31  34 
ricdnimte  3 
I'yrit45  1 


Pyroxene.s,  MoniM-linic  36 
Pyrojcenes,  OrtboritomUc  35 
PynUtg  :{ 
Pyrrhotite  1 

Quartz  5 

Ricbcckite  25,  36 
Rutile  1,5,  21 

Sagcnitc,  15,  21 

Sanidinc  9,  31 

Scaiwlitc  6 

Sencitc,  12,  18 

Serpentine  (Fibrous  variety,  Chryso- 

lite) 11 
Serpentine  (Leafy  variety,  see  Antigo- 

rite) 

Siilc-ritc  7,  23 
Silliniatiitc  11,  IS 
Soda  orthoclusc  30 
Sxxlalite  2 
SpcsHartitc  3 
Sphenc  (Sec  Titniuto 
Spinel,  Precious  3 
SiMNliiuieiie  11,  17,  26,  36 
Stiturolitc  26 
Still.ite  9 

Talc  13,  19 
Thulite  25 
Titanitc  13,  19,  28 
Titanolivine  27 
Topai  9 
Tourmaline  -- 
Tremolitc  12,  36 
Tridymite  5 

Uwarowite  3 
Vesuvianite  6,  15,  21 

\\  erncrite  6 
WoUaatonitc  11.35 

Zinnwiildite  28 
Zircon  6,  15 
Zoisite  10,  17 


PUHTED  IN  TBX  UJ.A. 

53 


De  irst,  de  geiht, 

Dit  is  de  tweit'; 

Will  wiinschen  dat  de't  ok  noch  deiht. 

Un  wenn  hei't  dauhn  deiht,  kann  hei  gahn, 

Ick  heww  an  em  dat  Minig  dahn. 

Wenn  Einer  dauhn  deiht,  wat  hei  deiht, 

Denn  kann  hei  nich  mihr  dauhn,  as  hei  deiht. 

FRITZ  REUTER 


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